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By Authority Of 

THE UNITED STATES OF AMERICA 

Legally Binding Document 



By the Authority Vested By Part 5 of the United States Code § 552(a) and 
Part 1 of the Code of Regulations § 51 the attached document has been duly 
INCORPORATED BY REFERENCE and shall be considered legally 
binding upon all citizens and residents of the United States of America. 
HEED THIS NOTICE : Criminal penalties may apply for noncompliance. 



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Document Name: aashto Green: A Policy on Geometric 

Design of Highways and Streets (2001) 
CFRSection(s): 23 cfr 625.4 

Standards Body: American Association of State Highway 

and Transportation Officials 



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^ POilCY ON GEOMETRIC DESIGN OF 




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HIGHWAYS AND STREETS 



2001 




FOURTH EDITION 

AMERICAN ASSOCIATION OF STATE HIGHWAY 
AND TRANSPORTATION OFFICIALS 




AMERICAN ASSOCIATION OF STATE HIGHWAY 
AND TRANSPORTATION OFFICIALS 





JM 



'/ I 




444 NORTH CAPITOL STREET, N.W., SUITE 249 

WASHINGTON, D.C. 20001 

www.transportation.org 




ISBN 1-56051-156-7 
BOOK CODE GDHS-4 




A POLICY on 

GEOMETRIC DESIGN of 

HIGHWAYS and STREETS 



2001 

Second Prietiiig 




American Association of State 

Highway aed Transportation Officials 

444 North Capitol Street, N.W,, Seite 249 

Washington, B.C. 20001 

(202) 624-5800 

wwwetraiisportatioe.org 



"^Copyright 2001, by the American Association of State Highway and 
Transportation Officials. All Rights Reserved. This book, or parts 
thereof, may not be reproduced in any form without written permission 
of the publisher. Printed in the United States of America. 

ISBN: 1-56051-156-7 



American Association of State Highway and 

Transportation Officials 

Executive Committee 

2000-2001 



President: Thomas R. Wame, Utah 

Vice President: Dean Carlson, Kansas 

Secretary/Treasurer: Larry King, Pennsylvania 

AASHTO Executive Director: John Horsley, Washington, D.C. 

Immediate Past President: Dan Flowers, Arkansas 

Regional Representatives 

Region I: William Ankner, Rhode Island — President 

(NASTO) Joseph Boardman, New York — Vice President 

Region II: David McCoy, North Carolina — President 

(SASHTO) Bruce Saltsman, Tennessee — ^Vice President 

Region III: Cristine Klika, Indiana — President 

(Mississippi Valley) Kirk Brown, Illinois — Vice President 

Region IV: Joseph Perkins, Alaska— President 

(WASHTO) Neal McCaleb, Oklahoma— Vice President 



n 



TASK FORCE ON GEOMETRIC DESIGN 2000 

Members 



Terry L. Abbott 


California 


1999-2000 


Don T. Arkle 


Alabama 


1991 -Present 


Ray Ballentine 


Mississippi 


1997-1999 


Harold E. Bastin 


National League of Cities 


1993 » 1999 


Paul Bercich 


Wyoming 


1995 -Present 


James 0. Brewer 


Kansas 


1986 -Present 


Jerry Champa 


California 


1997 - 1999 


Philip J. Clark 


New York 


1992 -Present 


Susan Davis 


Oklahoma 


1994-1995 


Alan Glenn 


California 


1992-1997 


Charles A. Goessel 


New Jersey 


1986 -Present 


Dennis A. Grylicki 


National Association of County Engineers 


1992-1999 


Irving Harris 


Mississippi 


1992-1997 


David Hutchison 


National Ixague of Cities 


1999 -Present 


John LaPlante 


American Public Works Association 


1989 -Present 


Ken T ,a7,ar 


Illinois 


1990-2000 


Donald A. Lyford 


New Hampshire 


1992 -Present 


Mark A. Marek 


Texas 


1986 - Present 


Terry H. Ottemess 


Arizona 


1997 -Present 


Steven R. Oxoby 


Nevada 


1993 -Present 


Robert P. Parisi 


Port Authority of New York and New Jersey 


1992 -Present 


Randy Peters 


Nebraska 


1993 - 1998 


John Pickering 


Mississippi 


1999 -Present 


William A. Prosser 


FHWA, Secretary 


1995 -Present 


Norman H. Roush 


West Virginia 


1979 -Present 


Joe Ruffer 


National Association of County Engineers 


1999 -Present 


John Sacksteder 


Kentucky 


1991-2000 


Larry Sutherland 


Ohio 


1991 -Present 


Charlie V. Trujillo 


New Mexico 


1998 -Present 


Robert L. Walters 


Arkansas, Chairman 


1982 -Present 


Ted Watson 


Nebraska 


1998 -Present 



Reza Amini 
Don T. Arkle 
Paul Bercich 
James O. Brewer 
Philip J. Clark 
Ron Erickson 
Charles A. Goessel 
David Hutchison 
Jeff C. Jones 
Wayne Kinder 
John LaPlante 
Donald A. Lyford 
Mark A. Marek 
Robert P. Parisi 
John Pickering 
William A. Prosser 
Norman H. Roush 
Joe Ruffer 
Larry Sutherland 
KarlaSutliff 
Charlie V. Trujillo 
Robert L. Walters 
Ted Watson 



TASK FORCE ON GEOMETRIC DESIGN 2002 
Members 

Oklahoma 

Alabama 

Wyoming 

Kansas 

New York 

Minnesota 

New Jersey 

National League of Cities 

Tennessee 

Nevada 

American Public Works Association 

New Hampshire 

Texas 

Port Authority of New York and New Jersey 

Mississippi 

Federal Highway Administration, Secretary 

West Virginia 

National Association of County Engineers 

Ohio 

California 

New Mexico 

Arkansas, Chair 

Nebraska 



III 



AASHTO Highway Subcommittee on Design 

2000-2001 

Dr. Kam K. Movassaghi, LOUISIANA, Chair 

Susan Martinovich, NEVADA, Vice-Chair 

Dwight A. Home, FHWA, Secretary 

Ken F. Kobetsky, P.E., AASHTO, Staff Liaison 



ALABAMA-- Arkle, Don T. 

Chief, Design Bureau 
Alabama Department of Transportation 
1409 Coliseum Boulevard 
Montgomery, AL 36 1 30-3050 
ALABAMA— Walker, Steven E. 

Assistant Chief Design Engineer 
Alabama Department of Transportation 
1409 Coliseum Boulevard 
Montgomery, AL 36130-3050 
ALASKA— Hogins, Gary 

Chief of Design & Construction 

Standards 
Alaska Department of Transportation & 

Public Facilities 
3132 Channel Drive 
Juneau, AK 99801 -7898 
ARIZONA— Louis, John L. 

Assistant State Engineer, Roadway Group 
Arizona Department of Transportation 
205 South 17th Ave., Mail Drop 61 IE 
Phoenix, AZ 85007-3213 
ARKANSAS— Loe, Dale F. 

Assistant Chief Engineer — Design 
Arkansas State Highway & Transportation 

Department 
P.O. Box 2261, 10324 Interstate 30 
State Highway Building 
Little Rock, AR 72203-2261 
ARKANSAS— McConnell, Phillip L. 

Engineer of Roadway Design 

Arkansas State Highway & Transportation 

Department 
P.O. Box 2261, 10324 Interstate 30 
State Highway Building 
Little Rock, AR 72203-2261 
CALIFORNIA— Buckley, Robert L. 

State and Local Project Development 
Program Manager 

California Department of Transportation 
P.O. Box 942874, 1 120 N Street 
Sacramento, CA 94273 
COLORADO— Harris, Timothy J. 

Project Development Branch Manager 
Colorado Department of Transportation 
4201 East Arkansas Avenue, Room 406 
Denver, CO 80222 
CONNECTICUT— Bard, Cari F. 

Principal Engineer 

Connecticut Department of Transportation 
P.O. Box 317546/2800 Berlin Turnpike 
Newington, CT 06 1 3 1 -7546 



CONNECTICUT— Byrnes, James F. 
Chief Engineer 

Connecticut Department of Transportation 
P.O. Box 317546/2800 Berlin Turnpike 
Newington, CT 06 1 3 1 -7546 
CONNECTICUT— Smith, Bradley J. 

Manager of State Design 
Connecticut Department of Transportation 
P.O. Box 317546/2800 Berlin Turnpike 
Newington, CT 06131-7546 

DELAWARE— Angelo, Michael A. 

Assistant Director, Design Support 
Delaware Department of Transportation 
P.O. Box 778, Bay Road, Route 1 13 
Dover, DEI 9903-0778 

DELAWARE— Canning, Kevin 

Supervising Engineer — Road Design 
Delaware Department of Transportation 
P.O. Box 778, Bay Road, Route 113 
Dover, DE 19903-0778 

DELAWARE— Satterfield, Joe 

Specifications Engineer 
Delaware Department of Transportation 
P.O. Box 778, Bay Road, Route 1 1 3 
Dover, DE 19903-0778 

DELAWARE— Simmons, Michael H. 
Road Design Engineer 
Delaware Department of Transportation 
P.O. Box 778, Bay Road, Route 113 
Dover, DE 19903-0778 
FLORIDA— Hattaway, Billy L. 

State Roadway Design Engineer 
Florida Department of Transportation 
605 Suwannee Street 
Tallahassee, FL 32399-0450 
FLORIDA— Mills, Jim 

Roadway Design Engineer 
Florida Department of Transportation 
605 Suwannee Street 
Tallahassee, FL 32399-0450 
FLORIDA— Simmons, Freddie L. 

State Highway Engineer 
Florida Department of Transportation 
605 Suwannee Street, MS 38 
Tallahassee, FL 32311-0450 
GEORGIA— Kennerly, James 

State Road and Airport Design Engineer 
Georgia Department of Transportation 
2 Capitol Square, Room 444 
Atlanta, GA 30334 



IV 



GEORGIA— Palladi, Joseph 

State Urban and Multi-Modal Design 

Engineer 
Georgia Department of Transportation 
No. 2 Capitol Square, Room 356 
Atlanta, GA 30334 
GEORGIA— Scott, Walker W. 

Georgia Department of Transportation 
2 Capitol Square 
Atlanta, GA 30334 
HAWAII— Abe, Casey 

Engineer Program Manager, Design 

Branch, Highways Division 
Hawaii Department of Transportation 
601 Kamokila Boulevard, Room 688 A 
Kapolei, HI 96707 
HAWAII— Fronda, Juhus 

Highway Design Section Head 
Hawaii Department of Transportation 
601 Kapolei Boulevard, Room 609 
Kapolei, HI 96707 
IDAHO— Hutchinson, Steven C. 

Assistant Chief Engineer — Development 
Idaho Transportation Department 
P.O. Box 7129, 3311 W. State Street 
Boise, ID 83707 
IDAHO — Laragan, Gregory 

Roadway Design Engineer 
Idaho Transportation Department 
P.O. Box 7 129, 3311 W. State Street 
Boise, ID 83707 
ILLINOIS— Hine, Michael 

Chief of Design and Environment 
Illinois Department of Transportation 
2300 S. Dirksen Parkway 
Springfield, IL 62764 
ILLINOIS— Seyfried, Robert 

Northwestern University Center for 

Pubhc Safety 
405 Church Street 
Evanston, IL 60204 
INDIANA— Klika, Phelps H. 

Director, Division of Design 
Indiana Department of Transportation 
100 N. Senate Avenue 
Indianapolis, IN 46204-2217 
IOWA— Dillavou, Mitch 

Director, Office of Design 
Iowa Department of Transportation 
800 Lincoln Way 
Ames, lA 50010 
IOWA— Little, David 

Deputy Director 

Iowa Department of Transportation, 

Engineering Division 
800 Lincoln Way 
Ames, lA 50010 
KANSAS— Adams, Richard G. 

Road Design Engineer 
Kansas Department of Transportation 
915 Harrison Ave., 9th Floor 
Topeka,KS 66612-1568 
KANSAS— Brewer, James O. 

Engineering Manager — State Road Office 
Kansas Department of Transportation 
Docking State Office Building, 9th Floor 
Topeka,KS 66612-1568 



KENTUCKY— Kratt, David 

Location Branch Manager 
Kentucky Transportation Cabinet, 

Division Of Highway Design 
High and Clinton streets, 6th Floor 
Frankfort, KY 40622 
KENTUCKY— Sperry, Kenneth R. 

Assistant State Highway Engineer 
Kentucky Transportation Cabinet, State 

Highway Engineer's Office 
501 High Street, State Office Building 
Frankfort, KY 40622 
LOUISIANA— Israel, N. Kent 

Roadway Design Engineer Administrator 
Louisiana Department of Transportation 

and Development 
P.O. Box 94245, 1201 Capitol Access Road 
Baton Rouge, LA 70804-9245 
LOUISIANA— Kalivoda, Nicholas 

Traffic and Geometries Design Engineer 
Louisiana Department of Transportation 

and Development 
Trenton, LA 86250 
LOUISIANA— Porta, Lloyd E. 
Design Squad 
Louisiana Department of Transportation 

and Development 
P.O. Box 94245, 1201 Capitol Access Road 
Baton Rouge, LA 70804-9245 
MAINE— Casey, Jerry A. 

Program Manager — Urban and Arterial 

Highways 
Maine Department of Transportation 
Transportation Building, State House 
Station 16 

Augusta, ME 04333-0016 
MARYLAND— Douglass, Robert D. 

Deputy Chief Engineer-Highway 

Development 
Maryland Department of 

Transportation, State Highway 

Administration 
707 N. Calvert Street, Mail Stop C102 
Baltimore, MD 21202 
MARYLAND— McClelland, Kirk G. 

Highway Design Division Chief 
Maryland Department of 

Transportation, State Highway 

Administration 
707 N. Calvert Street 
Baltimore, MD 21202 
MASSACHUSETTS— Blundo, John 

Deputy Chief Engineer, Highway 

Engineering 
Massachusetts Highway Department 
10 Park Plaza, Room 6340 
Boston, MA 02116-3973 
MASSACHUSETTS— Wood, Stanley 

Highway Location and Design Engineer 
Massachusetts Highway Department 
10 Park Plaza 
Boston, MA 021 16 
MICHIGAN— Miller, Paul F. 

Engineer of Design 

Michigan Department of Transportation, 

Design Division 
State Transportation Building 
425 W. Ottawa Street, P.O. Box 30050 
Lansing, MI 48909 



MINNESOTA— Gerdes, Delbert 

Director, Technical Support 
Minnesota Department of Transportation 
Transportation Building, MS 675, 395 
John Ireland Boulevard 
St, Paul, MN 55155-1899 
MISSISSIPPI— Pickering, John B. 

Roadway Design Engineer 
Mississippi Department of Transportation 
P.O. Box 1850, 401 North West Street 
Jackson, MS 39215-1850 
MISSISSIPPI— Ruff, Wendel T. 

Assistant Chief Engineer — Preconstruction 
Mississippi Department of Transportation 
P.O. Box 1850, 401 North West Street 
Jackson, MS 39215-1850 
MISSOURI— Nichols, David B. 

Director of Project Development 
Missouri Department of Transportation 
P.O. Box 270 

Jefferson City, MO 65102-0207 
MISSOURI— Yamell, William (Bill) 

Division Engineer Design 
Missouri Department of Transportation 
105 West Capitol Avenue, P.O. Box 270 
Jefferson City, MO 65 1 02-0207 
MONTANA— Peil, Carl S. 

Preconstruction Engineer 
Montana Department of Transportation 
P.O. Box 201001, 2701 Prospect Avenue 
Helena, MT 59620-1001 
NEBRASKA— Poppe, Eldon D. 

Engineer, Roadway Design Division 
Nebraska Department of Roads 
1500 Nebraska Highway 2 
P.O. Box 94759 
Lincoln, NE 68509-4759 
NEVADA— Oxoby, Steve R. 

Chief Road Design Engineer 
Nevada Department of Transportation 
1263 S.Stewart Street 
Carson City, NV 89712 
NEW HAMPSHIRE— Green, Craig A. 

Administrator, Bureau of Highway Design 
New Hampshire Department of 

Transportation 
John O. Morton Building, P.O. Box 483 
1 Hazen Drive 
Concord, NH 03301-0483 

NEW JERSEY— Dunne, Richard W. 

Director, Design Services 
New Jersey Department of Transportation 
1035 Parkway Avenue, CN 600 
Trenton, NJ 08625-0600 

NEW JERSEY— Eisdorfer, Arthur J. 

Manager, Bureau of Civil Engineering 
New Jersey Department of Transportation 
1035 Parkway Avenue, CN 600 
Trenton, NJ 08625-0600 

NEW JERSEY— Miller, Charles 

Executive Assistant, Office of the Director 
New Jersey Department of Transportation, 

Division Of Design Services 
1035 Parkway Avenue, CN 600 
Trenton, NJ 08625-0600 



NEW MEXICO— Maestas, Roy 

Chief, Internal Design Bureau 
New Mexico State Highway and 

Transportation Department 
P.O. Box 1 149, 1 120 Cerrillos Road 
Santa Fe,NM 87504-1 149 
NEW MEXICO— Trujillo, Charhe V. 

Deputy Secretary of Transportation 

Planning and Design 
New Mexico State Highway and 
Transportation Department 
P.O. Box 1 149, 1 120 Cerrillos Road 
Santa Fe,NM 87504-1 149 
NEW YORK— Bellair, Peter J. 

Director of Design Quality Insurance 

Bureau 
New York Department of Transportation 
Building 5, State Office Campus 
1 220 Washington Avenue 
Albany, NY 12232-0750 
NEW YORK— Clark, Phillip J. 

Deputy Chief Engineer/Director, Design 

Division 
New York Department of Transportation 
Building 5, State Office Campus 
1 220 Washington Avenue 
Albany, NY 12232-0748 
NEW YORK— D'Angelo, Daniel 

Director, Design QuaHty Assurance Bureau 
New York Department of Transportation 
1220 Washington Ave. 
Building 5, Room 410 
Albany, NY 12232-0751 
NORTH CAROLINA— Alford, John E. 

State Roadway Design Unit 
North Carolina Department of 

Transportation 
P.O. Box 25201, 1 South Wilmington Street 
Raleigh, NC 2761 1-5201 
NORTH CAROLINA— Barbour, Deborah M. 
State Design Engineer 
North Carolina Department of 

Transportation 
P.O. Box 25201, 1 South Wilmington Street 
Raleigh, NC 2761 1-5201 
NORTH CAROLINA— Hill, Len 

Deputy Administrator, Pre-Construction 
North Carolina Department of 

Transportation 
P.O. Box 25201, 1 South Wilmington Street 
Raleigh, NC 2761 1-5201 
NORTH CAROLINA— Morton, Don R. 

Deputy Administrator — Preconstruction 
North Carolina Department of 

Transportation, Division of Highways 
P.O. Box 25201, 1 South Wilmington Street 
Raleigh, NC 2761 1-5201 
NORTH DAKOTA— Birst, Kenneth E. 
Design Engineer 

North Dakota Department of Transportation 
608 E. Boulevard Avenue 
Bismarck, ND 58505-0700 
OHIO— Misel, Cash 

Assistant Director and Chief Engineer 
Ohio Department of Transportation, 

Planning and Production Management 
1980 West Broad Street 
Columbus, OH 43223-1102 



VI 



OHIO— Sutherland, Larry F. 

Deputy Director, Office of Roadway 

Engineering Services 
Ohio Department of Transportation 
1980 West Broad Street 
Columbus, OH 43223 
OKLAHOMA— Senkowski, Christine M. 

Division Engineer, Roadway Design 
Oklahoma Department of Transportation 
200 N. E. 21st Street, Room 2c-2 
Oklahoma City, OK 73105-3204 
OKLAHOMA— Taylor, Bruce E. 
Chief Engineer 

Oklahoma Department of Transportation 
200 N.E. 21st Street 
Oklahoma City, OK 73105-3204 
OREGON— Greenberg, Dave 

Design Unit Manager 
Oregon Department of Transportation 
355 Capitol Street N.E, Room 200 
Salem, OR 97310 
OREGON— Nelson, Catherine 

Manager, Roadway Engineering Section 
Oregon Department of Transportation 
200 Transportation Building 
Salem, OR 97310 
OREGON— Scheick, Jeff 

Manager, Technical Services 
Oregon Department of Transportation 
Transportation Building, 355 Capitol Street 
Salem, OR 97310 
PENNSYLVANIA— Schreiber, Dean A. 

Chief, Highway QuaHty Assurance Div. 
Pennsylvania Department of Transportation 
P.O. Box 3161 
Harrisburg, PA 17105-3161 
PUERTO RICO— Hernandez Borges, Jose E. 
Director, Design Area 
Puerto Rico Highway any Transportation 

Authority 
P.O. Box 42007, Minillas Station 
San Juan, PR 00940-2007 
RHODE ISLAND— Bennett, J. Michael 

Managing Engineer, Highway Design 
Rhode Island Department of Transportation 
State Office Building, 2 Capitol Hill 
Providence, RI 02903-1 124 
SOUTH CAROLINA— Kneece, Rocque L. 
C Fund Manager 
South Carolina Department of 

Transportation 
Silas N. Pearman Building, 955 Park Street 
Box 191 

Columbia, SC 29202-0191 
SOUTH CAROLINA— Pratt, Robert 1. 

Project Development Engineer 
South Carolina Department of 

Transportation 
Silas N. Pearman Building, 955 Park Street 
Box 191 

Columbia, SC 29202-0191 
SOUTH CAROLINA— Walsh, John V. 

Program Development Engineer 
South Carolina Department of 

Transportation 
Silas N. Pearman Building, 955 Park Street 
Box 191 
Columbia, SC 29202-0191 



SOUTH DAKOTA— Bjomeberg, Timothy 

Chief Road Design Engineer 
South Dakota Department of Transportation 
700 East Broadway Avenue 
Pierre, SD 57501-2586 
SOUTH DAKOTA— Feller, Joe 

Chief Materials and Surfacing Engineer 
South Dakota Department of Transportation 
700 East Broadway Avenue 
Pierre, SD 57501-2586 
TENNESSEE— Jones, Jeff C. 

Civil Engineer Director, Design Division 
Tennessee Department of Transportation 
505 Deaderick Street, Suite 700 
Nashville, TN 37243-0339 
TENNESSEE— Zeigler, James 

Director, Bureau of Planning and 

Development 
Tennessee Department of Transportation 
700 James K. Polk Building, 
Fifth and Deaderick 
Nashville, TN 37243-0339 
TEXAS— Wilson, Robert L. 
Director, Design 

Texas Department of Transportation 
125 East 11th Street 
Austin, TX 78701-2483 
UTAH— Mohanty, P. K. 

Roadway Design Engineer 
Utah Department of Transportation 
4501 South 2700 West 
Salt Lake City, UT 841 19 
VERMONT— Lathrop, Donald H. 

Plan Support Engineer 
Vermont Agency of Transportation 
State Administration Building 
133 State Street 
MontpeHer, VT 05633-5001 
VERMONT— Shattuck, Robert F. 

Roadway and Traffic Design Program 

Manager 
Vermont Agency of Transportation 
State Administration Building 
133 State Street 
Montpelier,VT 05633-5001 
VIRGINIA— Harris, James T. 

Assisant Division Administrator 
Virginia Department of Transportation, 

Location and Design Division 
1401 E. Broad Street 
Richmond, VA 23219 
VIRGINIA— Mills, Jimmy 

Location and Testing Engineer 
Virginia Department of Transportation 
1401 East Broad Street 
Richmond, VA 23219 
VIRGINIA— Mirshahi, Mohammad 

Assistant Division Administrator 
Virginia Department of Transportation 
1401 E. Broad Street 
Richmond, VA 23219 
WASHINGTON— Albin, Richard 

Standards Engineer 
Washington State Department of 

Transportation 
Transportation Building 
310 Maple Park, P.O. Box 47329 
Olympia, WA 98504-7329 



Vll 



WASHINGTON— Ziegler, Brian J. 

State Design Engineer 
Washington State Department of 

Transportation 
505 Deaderick Street, Suite 700 
Olympia, WA 98504-7300 
WEST VIRGINIA— Clevenger, David E. 

Consultant Review Section Head 
West Virginia Department of 

Transportation, Engineering Division 
1900 Kanawha Boulevard East, Building 5 
Charleston, WV 25305-0440 
WEST VIRGINIA— Epperiy, Randolph T. 

Deputy State Highway Engineer-Project 

Development 
West Virginia Department of 

Transportation 
1900 Kanawha Boulevard East, Building 5 
Charleston, WV 25305-0440 
WEST VIRGINIA— Roush, Norman H. 

Deputy Commissioner of Highways 
West Virginia Department of 

Transportation 
1900 Kanawha Boulevard East, Building 5 
Charleston, WV 25305-0440 
WISCONSIN— Haverberg, John E. 

Director, Bureau of Highway Development 
Wisconsin Department of Transportation 
P.O. Box 7910 
4802 Sheboygan Avenue 
Madison, WI 53707-7910 
WISCONSIN— Pfeiffer, Robert F. 

Project Development Chief 

Wisconsin Department of Transportation, 

District 2, Waukesha 

P.O. Box 7910 

4802 Sheboygan Avenue 

Madison, WI 53707-7910 



WYOMING— Bercich, Paul 

Project Development Engineer 
Wyoming Department of Transportation 
P.O. Box 1708, 5300 Bishop Boulevard 
Cheyenne, WY 82003-1708 
DISTRICT OF 
COLUMBIA— Rice, John 

Manager, Engineering and Specifications 

Division 
Federal Aviation Administration 
800 Independence Avenue, S.W. 
Room 616C, AAS-200 
Washington, D.C. 20591-0001 
DISTRICT OF 
COLUMBIA— Sandhu, Harbhajan S. 

Chief, Design and Engineering Division 
District of Columbia Department of Public 

Works 
2000 14th Street, N.W., 5th Floor 
Washington, D.C. 20009 
BRITISH COLUMBIA, 

CANADA— Voyer, Richard 

Senior Standards and Design Engineer 
British Columbia Ministry of 

Transportation and Highways 
5B - 940 Blanshard Street 
Victoria, British Columbia V8W 3E6 



Vlll 



Preface 



This Policy was developed as part of the continuing work of the Standing Committee on 
Highways. The Committee, then titled the Committee on Planning and Design Policies, was 
established in 1937 to formulate and recommend highway engineering policies. This Committee 
has developed A Policy on Geometric Design of Rural Highways, 1954 and 1965 editions; A 
Policy on Arterial Highways in Urban Areas, 1957; A Policy on Design of Urban Highways and 
Arterial Streets, 1973; Geometric Design Standards for Highways Other Than Freeways, 1969; A 
Policy on Geometric Design of Highways and Streets, 1984, 1990, and 1994; A Policy on Design 
Standards — Interstate System, 1956, 1967, and 1991; and a number of other AASHO and 
AASHTO policy and "guide" publications. 

An AASHTO pubhcation is typically developed through the following steps: (l)The 
Committee selects subjects and broad outlines of material to be covered. (2) The appropriate 
subcommittee and its task forces, in this case, the Subcommittee on Design and its Task Force on 
Geometric Design, assemble and analyze relevant data and prepare a tentative draft. Working 
meetings are held and revised drafts are prepared, as necessary, and reviewed by the 
Subcommittee, until agreement is reached. (3) The manuscript is then subnnitted for approval by 
the Standing Committee on Highways and then the Executive Committee. Standards and policies 
must be adopted by a two-thirds vote by the Member Departments before publication. During the 
developmental process, comments are sought and considered from all the states, the Federal 
Highway Administration, and representatives of the American Public Works Association, the 
National Association of County Engineers, the National League of Cities, and other interested 
parties. 



Notice to User 



This second printing of the Fourth Edition of AASHTO' s A Policy on Geometric Design of 
Highways and Streets incorporates the revisions that were approved by the Task Force on 
Geometric Design during the meeting of July 2002. To view a complete list of the changes that 
were made, please visit AASHTO' s online bookstore at www.transportation.org. 



IX 



Table of Contents 



Preface ix 

Foreword xv 

CHAPTER TITLES 

Chapter 1 Highway Functions 1 

Chapter 2 Design Controls and Criteria 15 

Chapter 3 Elements of Design 109 

Chapter 4 Cross Section Elements 309 

Chapter 5 Local Roads and Streets 383 

Chapter 6 Collector Roads and Streets 423 

Chapter 7 Rural and Urban Arterials 447 

Chapter 8 Freeways.. 507 

Chapter 9 Intersections 559 

Chapter 10 Grade Separations and Interchanges 747 

Chapter 1 

HIGHWAY FUNCTIONS 

Page 

Systems and Classifications 1 

The Concept of Functional Classification 1 

Hierarchies of Movements and Components 1 

Functional Relationships 4 

Access Needs and Controls 6 

Functional System Characteristics 7 

Definitions of Urban and Rural Areas 7 

Functional Categories. 8 

Functional Systems for Rural Areas 8 

Rural Principal Arterial System 8 

Rural Minor Arterial System 9 

Rural Collector System 9 

Rural Local Road System 9 

Extent of Rural Systems 10 

Functional Highway Systems In Urbanized Areas 10 

Urban Principal Arterial System 10 

Urban Minor Arterial Street System 11 

Urban Collector Street System 12 

Urban Local Street System 12 

Length of Roadway and Travel on Urban Systems 12 

Functional Classification as a Design Type 13 

References 14 

xi 



Chapter 2 

DESIGN CONTROLS AND CRITERIA 

Introduction 15 

Design Vehicles 15 

General Characteristics.. 15 

Minimum Turning Paths of Design Vehicles 18 

Vehicle Performance 43 

Vehicular Pollution 43 

Driver Performance 46 

Introduction 46 

Older Drivers 47 

The Driving Task..... 47 

The Guidance Task 48 

Lane Placement and Road Following 48 

Car Follov^ing 48 

Passing Maneuvers 49 

Other Guidance Activities 49 

The Information System 49 

Traffic Control Devices 49 

The Roadway and its Environment 49 

Information Handling 50 

Reaction Time....... 50 

Primacy 53 

Expectancy 53 

Driver Error 53 

Errors Due to Driver Deficiencies 54 

Errors Due to Situation Demands 56 

Speed and Design 56 

Design Assessment 57 

Traffic Characteristics 58 

General Considerations 58 

Volume 58 

Average Daily Traffic 58 

Peak-Hour Traffic... 59 

Directional Distribution 62 

Composition of Traffic..,. 63 

Projection of Future Traffic Demands 65 

Speed 66 

Operating Speed .....66 

Running Speed 67 

Design Speed 67 

Traffic Flow Relationships 72 

Highway Capacity 74 

General Characteristics 74 

Application 74 

xii 



Capacity as a Design Control..... 75 

Design Service Flow Rate Versus Design Volume 75 

Measures of Congestion 75 

Relation Between Congestion and Traffic Flow Rate 76 

Acceptable Degrees of Congestion 77 

Principles for Acceptable Degrees of Congestion 78 

Reconciliation of Principles for Acceptable Degrees of Congestion 80 

Factors Other Than Traffic Volume That Affect Operating Conditions 81 

Highway Factors 81 

Alignment 82 

Weaving Sections.. 82 

Ramp Terminals 82 

Traffic Factors 83 

Peak Hour Factor.... 83 

Levels of Service 84 

Design Service Flow Rates 85 

Weaving Sections 85 

Multilane Highways Without Access Control 86 

Arterial Streets and Urban Highways 86 

Intersections 88 

Pedestrians and Bicycles 88 

Access Control and Access Management 88 

General Conditions 88 

Basic Principles of Access Management 90 

Access Classifications 90 

Methods of Controlling Access 91 

Benefits of Controlling Access 91 

The Pedestrian 96 

General Considerations 96 

General Characteristics 96 

Walking Speeds 97 

Walkway Capacities , 98 

Sidewalks 98 

Intersections 99 

Reducing Pedestrian-Vehicular Conflicts 99 

Characteristics of Persons With Disabilities , 99 

Mobility Impairments 100 

Visual Impairments 100 

Developmental Impairments 100 

Bicycle Facilities 100 

Safety 101 

Environment 106 

Economic Analysis 106 

References 106 



Xlll 



Chapter 3 

ELEMENTS OF DESIGN 

Introduction 109 

Sight Distance 109 

General Considerations 109 

Stopping Sight Distance 110 

Brake Reaction Time 110 

Braking Distance Ill 

Design Values 113 

Effect of Grade on Stopping 113 

Variation for Trucks 114 

Decision Sight Distance 115 

Passing Sight Distance for Two-Lane Highways 118 

Criteria for Design 118 

Design Values 122 

Effect of Grade on Passing Sight Distance 125 

Frequency and Length of Passing Sections 125 

Sight Distance for Multilane Highways 126 

Criteria for Measuring Sight Distance 127 

Height of Driver's Eye 127 

Height of Object 127 

Sight Obstructions 128 

Measuring and Recording Sight Distance on Plans 128 

Horizontal Alignment 131 

Theoretical Considerations 131 

General Considerations 132 

Superelevation 132 

Side Friction Factor 133 

Distribution of e and f Over aRange of Curves..... 138 

Design Considerations 141 

Maximum Superelevation Rates 141 

Minimum Radius 142 

Design for Rural Highways, Urban Freeways, and High-Speed Urban Streets 143 

Procedure for Development of Finalized e Distribution 146 

Design Superelevation Tables 155 

Sharpest Curve Without Superelevation 166 

Effects of Grades 167 

Transition Design Controls 168 

General Considerations 168 

Tangent-to-Curve Transition 169 

Spiral Curve Transitions 176 

Length of Spiral 177 

Compound Curve Transition 184 

Methods of Attaining Superelevation 184 

Design of Smooth Profiles for Traveled Way Edges 187 

xiv 



Axis of Rotation with a Median...... 188 

Minimum Transition Grades. 190 

Turning Roadway Design 191 

Design for Low-Speed Urban Streets 192 

Maximum Comfortable Speed on Horizontal Curves 195 

Minimum Superelevation Runoff Length 195 

Minimum Radii and Minimum Lengths of Superelevation Runoff for 

Limiting Values of ^ and/ 198 

Curvature of Turning Roadways and Curvature at Intersections 198 

Minimum Radius for Turning Speed 198 

Transitions and Compound Curves 203 

Length of Spiral 203 

Compound Circular Curves 205 

Offtracking 206 

Derivation of Design Values for Widening on Horizontal Curves 206 

Traveled Way Widening on Horizontal Curves 212 

Design Values for Traveled Way Widening 214 

Application of Widening on Curves 218 

Widths for Turning Roadways at Intersections 220 

Design Values 223 

Widths Outside Traveled Way 226 

Sight Distance on Horizontal Curves 228 

Stopping Sight Distance 228 

Passing Sight Distance 232 

General Controls for Horizontal Alignment 233 

Vertical Alignment 235 

Terrain 235 

Grades 235 

Vehicle Operating Characteristics on Grades 236 

Control Grades for Design 239 

Critical Lengths of Grade for Design 242 

Climbing Lanes 247 

Climbing Lanes for Two-Lane Highways 247 

Climbing Lanes on Freeways and Multilane Highways 25 1 

Methods for Increasing Passing Opportunities on Two-Lane Roads 254 

Passing Lanes 254 

Turnouts.. 257 

Shoulder Driving 258 

Shoulder Use Sections 259 

Emergency Escape Ramps 259 

General 259 

Need and Location for Emergency Escape Ramps 261 

Types of Emergency Escape Ramps 262 

Design Considerations 264 

Brake Check Areas ..268 

Maintenance 268 

XV 



Vertical Curves ....269 

General Considerations 269 

Crest Vertical Curves 270 

Sag Vertical Curves 276 

Sight Distance at Undercrossings 280 

General Controls for Vertical Alignment 282 

Combinations of Horizontal and Vertical Alignment 283 

General Considerations 283 

General Design Controls.... 284 

Alignment Coordination in Design 285 

Other Elements Affecting Geometric Design 286 

Drainage 286 

Erosion Control and Landscape Development 292 

Rest Areas, Information Centers, and Scenic Overlooks 293 

Lighting 294 

Utilities 296 

General.. 296 

Urban 297 

Rural 298 

Traffic Control Devices 298 

Signing and Marking 298 

Traffic Signals.. 299 

Noise Barriers 300 

Fencing 301 

Maintenance of Traffic Through Construction Areas 301 

References 303 

Chapter 4 
CROSS SECTION ELEMENTS 

General 309 

Pavement 309 

Surface Type .....309 

Cross Slope 309 

Skid Resistance 314 

Lane Widths 315 

Shoulders 316 

General Characteristics..... 316 

Width of Shoulders. 318 

Shoulder Cross Sections 319 

Shoulder Stability 321 

Shoulder Contrast 322 

Turnouts 322 

Horizontal Clearance to Obstructions.... 322 

Curbs ....323 

General Considerations 323 

xvi 



Curb Configurations 324 

Curb Placement 326 

Drainage Channels and Sideslopes 327 

General Considerations 327 

Drainage Channels 327 

Sideslopes 330 

Illustrative Outer Cross Sections 333 

Normal Crown Sections 333 

Superelevated Sections 334 

Traffic Barriers 335 

General Considerations 335 

Longitudinal Barriers 337 

Roadside Barriers 337 

Median Barriers 338 

Bridge Railings 339 

Crash Cushions 340 

Medians 341 

Frontage Roads 343 

Outer Separations 346 

Noise Control 348 

General Considerations 348 

General Design Procedures 349 

Noise Reduction Designs 350 

Roadside Control 352 

General Considerations 352 

Driveways 352 

Mailboxes 354 

Tunnels 355 

General Considerations 355 

Types of Tunnels 356 

General Design Considerations 357 

Tunnel Sections 357 

Examples of Tunnels 359 

Pedestrian Facilities 361 

Sidewalks 361 

Grade-Separated Pedestrian Crossings 363 

Sidewalk Curb Ramps 365 

Bicycle Facilities 371 

Bus Turnouts 372 

Freeways , 372 

Arterials 372 

Park-and-Ride Facilities 374 

Location 374 

Design 375 

On-Street Parking 377 

References 380 

xvii 



Chapter 5 

LOCAL ROADS AND STREETS 

Introduction 383 

Local Rural Roads 384 

General Design Considerations 384 

Design Traffic Volume 384 

Design Speed 384 

Sight Distance 384 

Grades 386 

Alignment ....386 

Cross Slope 387 

Superelevation 387 

Number of Lanes 387 

Width of Traveled Way, Shoulder, and Roadway 387 

Structures 389 

Bridges to Remain in Place 389 

Vertical Clearance 389 

Right-of-Way Width 391 

Foreslopes 391 

Horizontal Clearance to Obstructions 391 

Curbs 392 

Intersection Design 392 

Railroad Highway Grade Crossings 392 

Traffic Control Devices 393 

Bicycle Facilities 393 

Erosion Control 393 

Local Urban Streets 393 

General Design Considerations 393 

Design Traffic Volume 394 

Design Speed 394 

Sight Distance 395 

Grades 395 

Alignment 395 

Cross Slope 396 

Superelevation 396 

Number of Lanes 396 

Width of Roadway 397 

Parking Lanes... 397 

Median 397 

Curbs 398 

Drainage 398 

Cul-De-Sacs and Turnarounds 398 

Alleys 400 

Sidewalks 400 

Sidewalk Curb Ramps 402 

xviii 



Driveways 402 

Roadway Widths for Bridges 403 

Horizontal Clearance to Obstructions 403 

Vertical Clearance 403 

Border Area 403 

Right-of-Way Width 404 

Provision for Utilities 404 

Intersection Design 404 

Railroad-Highway Grade Crossings 405 

Street and Roadway Lighting 406 

Levels 407 

Traffic Control Devices 407 

Erosion Control 407 

Landscaping 407 

Bicycle Facilities 408 

Special-Purpose Roads 408 

Introduction 408 

Recreational Roads.. 408 

General Considerations 408 

Design Speed 409 

Design Vehicle 410 

Sight Distance 410 

Passing Sight Distance 410 

Grades 411 

Vertical Alignment 413 

Horizontal Alignment 413 

Number of Lanes 415 

Widths of Traveled Way, Shoulder, and Roadway 415 

Cross Slope 415 

Clear Recovery Area 417 

Roadside Slopes 417 

Roadside Barrier 417 

Signing and Marking 418 

Structures 418 

Resource Recovery Roads 418 

Local Service Roads 420 

References 420 

Chapter 6 

COLLECTOR ROADS AND STREETS 

Introduction 423 

Rural Collectors..... 424 

General Design Considerations 424 

Design Traffic Volumes 424 

Design Speed 424 

xix 



Sight Distance 425 

Grades , 425 

Alignment 425 

Cross Slope 425 

Superelevation 428 

Number of Lanes 428 

Width of Roadway 428 

Foreslopes 428 

Structures 430 

Bridges to Remain in Place 430 

Vertical Clearance 431 

Horizontal Clearance to Obstructions 431 

Right-of-Way Width 432 

Intersection Design 432 

Railroad-Highway Grade Crossings 432 

Traffic Control Devices 433 

Erosion Control 433 

Urban Collectors 433 

General Design Considerations 433 

Design Traffic Volumes 434 

Design Speed 434 

Sight Distance 435 

Grades 435 

Alignment 435 

Cross Slope 435 

Superelevation 435 

Number of Lanes 437 

Width of Roadway 437 

Parking Lanes 437 

Medians 438 

Curbs. 439 

Drainage , 439 

Sidewalks 440 

Driveways. 440 

Roadway Widths for Bridges 440 

Vertical Clearance 440 

Horizontal Clearance to Obstructions 441 

Right-of-Way Width... 441 

Provision for Utilities 441 

Border Area 442 

Intersection Design 442 

Railroad-Highway Grade Crossings 443 

Street and Roadway Lighting 443 

Traffic Control Devices 443 

Erosion Control 444 

Landscaping 444 

XX 



References 444 

Chapter 7 

RURAL A^fD URBAN ARTERIALS 

Introduction 447 

Rural Arterials..... ....447 

General Characteristics 447 

General Design Considerations 448 

Design Speed 448 

Design Traffic Volume 448 

Levels of Service 448 

Sight Distance 449 

Alignment 449 

Grades. 450 

Number of Lanes 450 

Superelevation 450 

Cross Slope 450 

Vertical Clearances... 451 

Structures 451 

Traffic Control Devices 451 

Erosion Control 451 

Widths 452 

Horizontal Clearance to Obstructions 452 

Cross Section and Right-of-Way 453 

Provision for Passing 453 

Ultimate Development of Four-Lane Divided Arterials 454 

Multilane Undivided Arterials 457 

Divided Arterials 458 

General Features 458 

Lane Widths ,..459 

Cross Slope 459 

Shoulders 459 

Median Barrier Clearance 460 

Medians 460 

Alignment and Profile 461 

Climbing Lanes on Multilane Arterials 462 

Superelevated Cross Sections 463 

Cross Section and Right-of-Way Widths 466 

Sections With Widely Separated Roadways 469 

Intersections... 470 

Access Management 471 

Bikeways and Pedestrian Facilities.... 471 

Bus Turnouts 472 

Railroad-Highw^ay Crossings 472 

Rest Areas 472 

xxi 



Urban Arterials 473 

General Characteristics 473 

General Design Considerations 474 

Design Speed 474 

Design Traffic Volume 474 

Levels of Service 474 

Sight Distance 475 

Alignment 475 

Grades 475 

Superelevation 475 

Cross Slope 476 

Vertical Clearances 476 

Lane Widths 476 

Curbs and Shoulders 477 

Number of Lanes 477 

Width of Roadway , 478 

Medians 478 

Drainage 482 

Parking Lanes 482 

Borders and Sidewalks 483 

Railroad-Highway Crossings 484 

Roadway Width for Bridges 485 

Bridges to Remain in Place.. 485 

Horizontal Clearance to Obstructions 485 

Right-of-Way Width ....486 

Traffic Barriers 486 

Access Management 486 

General Features 486 

Access Control by Statute 487 

Access Control by Zoning 487 

Access Control Through Driveway Regulations 487 

Access Control Through Geometric Design 488 

Pedestrian Facilities 488 

Provision for Utilities 490 

Intersection Design 490 

Operational Control and Regulations 490 

Traffic Control Devices 490 

Regulatory Measures 492 

Operational and Control Measures for Right-Turn Maneuvers 492 

Operational and Control Measures for Left-Turn Maneuvers 492 

Regulation of Curb Parking 495 

Directional Lane Usage 495 

Frontage Roads and Outer Separations 498 

Grade Separations and Interchanges....... 498 

Erosion Control 500 

Lighting 500 

xxii 



Bikeways 500 

Public Transit Facilities 500 

Location of Bus Stops 501 

Bus Turnouts 504 

Reserved Bus Lanes 504 

Traffic Control Measures 505 

References 506 

Chapter 8 

FREEWAYS 

Introduction 507 

General Design Considerations 507 

Design Speed 507 

Design Traffic Volumes 508 

Levels of Service 508 

Pavement and Shoulders 508 

Curbs. 509 

Superelevation 509 

Grades... 509 

Structures 510 

Vertical Clearance 510 

Horizontal Clearance to Obstructions 511 

Ramps and Terminals 512 

Outer Separations, Borders, and Frontage Roads 512 

Rural Freev^ays 512 

Alignment and Profile 513 

Medians 513 

Sideslopes 516 

Frontage Roads 516 

Urban Freeways 517 

General Design Characteristics 517 

Medians 517 

Depressed Freeways 517 

General Characteristics..... 517 

Slopes and Walls 518 

Typical Cross Section 519 

Restricted Cross Section 521 

Walled Cross Section 521 

Examples of Depressed Freeways 522 

Elevated Freeways 524 

General Characteristics 524 

Medians 525 

Ramps and Terminals 525 

Frontage Roads , 526 

Clearance to Building Line 526 

xxiii 



Typical Cross Section 526 

Viaduct Freeways Without Ramps 527 

Two-Way Viaduct Freeways With Ramps 528 

Freeways on Earth Embankment 529 

Examples of Elevated Freeways 530 

Ground-Level Freeways 531 

General Characteristics 531 

Typical Cross Section 532 

Restricted Cross Section 533 

Example of a Ground-Level Freeway 534 

Combination-Type Freeways 534 

General Characteristics 534 

Profile Control 535 

Cross-Section Control... ...537 

Examples of Combination-Type Freeways 537 

Special Freeway Designs 541 

Reverse-Flow Roadways 541 

Dual-Divided Freeways 544 

Freeways With Collector-Distributor Roads 547 

Accommodation of Transit and High-Occupancy Vehicle Facilities 547 

General Considerations 547 

Buses 549 

Rail Transit 554 

References 558 

Chapter 9 

INTERSECTIONS 

Introduction 559 

General Design Considerations and Objectives 559 

Types and Examples of Intersections 562 

General Considerations 562 

Three-Leg Intersections 563 

Basic Types of Intersections 563 

Channelized Three-Leg Intersections 568 

Four-Leg Intersections 569 

Basic Types 569 

Channelized Four-Leg Intersections , 570 

Multileg Intersections 575 

Modem Roundabouts 578 

Capacity Analysis 583 

Alignment and Profile 583 

General Considerations 583 

Alignment , 584 

Profile...,. 586 

Types of Turning Roadways 587 

xxiv 



General 587 

Minimum Edge-of-Traveled-Way Designs 587 

Design for Specific Conditions (Right-Angle Turns) 596 

Passenger Vehicles 597 

Single-Unit Trucks and City Transit Buses 613 

Semitrailer Combination Trucks 614 

Oblique-Angle Turns 614 

Effect of Curb Radii on Turning Paths 615 

Effect of Curb Radii on Pedestrians 618 

Comer Radii Into Local Urban Streets 625 

Islands 625 

General Characteristics 625 

Channelizing Islands 627 

Divisional Islands 629 

Refuge Islands 630 

Island Size and Designation 631 

Island Delineation and Approach Treatment 632 

Turning Roadways With Comer Islands 638 

Right-Angle Turns With Comer Islands 638 

Oblique-Angle Turns With Comer Islands 641 

Free-Flow Turning Roadways at Intersections 643 

Superelevation for Turning Roadways at Intersections 643 

General Design Guidelines 643 

Superelevation Runoff 646 

Development of Superelevation at Turning Roadway Terminals 646 

General Procedure 647 

Turn-Lane Cross-Slope Rollover 652 

Superelevation Transition and Gradeline Control 652 

Traffic Control Devices 653 

Intersection Sight Distance 654 

General Considerations 654 

Sight Triangles 655 

Approach Sight Triangles 655 

Departure Sight Triangles 657 

Identification of Sight Obstmctions Within Sight Triangles 657 

Intersection Control 658 

Case A — Intersections With No Control 658 

Case B — Intersections With Stop Control on the Minor Road 660 

Case B 1— Left Turn From the Minor Road 660 

Case B2 — Right Turn from the Minor Road 667 

Case B3 — Crossing Maneuver from the Minor Road 667 

Case C — Intersections With Yield Control on the Minor Road 670 

Case CI — Crossing Maneuver From the Minor Road 670 

Case C2^Left- and Right-Turn Maneuvers 675 

Case D — Intersections With Traffic Signal Control 675 

CaseE — Intersections With All-Way Stop Control 678 



XXV 



Case F — ^Left Turns From the Major Road 678 

Effect of Skew 681 

Stopping Sight Distance at Intersections for Turning Roadways 682 

General Considerations 682 

Vertical Control .682 

Horizontal Control 683 

Design to Discourage Wrong-Way Entry 683 

General Intersection Types 686 

General Design Considerations 686 

Channelization 690 

Speed-Change Lanes at Intersections 692 

Median Openings 693 

General Design Considerations 693 

Control Radii for Minimum Turning Paths 694 

Shape of Median End 701 

Minimum Length of Median Opening 701 

Median Openings Based on Control Radii for Design Vehicles 702 

Passenger Vehicles 702 

Single-Unit Trucks or Buses 703 

Semitrailer Combinations 704 

Effect of Skew 704 

Above-Minimum Designs for Direct Left Turns 706 

Indirect Left Turns and U-turns 709 

General Design Considerations 709 

Indirect Left Turn or Indirect U-Tum — Using Local Streets 71 1 

Indirect Left Turn or Indirect U-Tum — Wide Medians 712 

Location and Design of U-Tum Median Openings 713 

Flush or Traversable Medians 716 

Auxihary Lanes 717 

General Design Considerations 717 

Deceleration Length 718 

Storage Length 718 

Taper 719 

Median Left-Turn Lanes 720 

Median End Treatment 726 

Offset Left-Turn Lanes 727 

Simultaneous Left Turns 727 

Intersection Design Elements with Frontage Roads 729 

Bicycles at Intersections 732 

Wheelchair Ramps at Intersections. 732 

Lighting at Intersections 733 

Driveways 733 

Railroad-Highway Grade Crossings 735 

Horizontal Alignment 735 

Vertical Alignment 735 

General 736 

xxvi 



References 743 

Chapter 10 

GRADE SEPARATIONS AND INTERCHANGES 

Introduction and General Types of Interchanges 747 

Warrants for Interchanges and Grade Separations 749 

Adaptability of Highway Grade Separations and Interchanges 751 

Traffic and Operation 751 

Site Conditions 752 

Type of Highway and Intersecting Facility 752 

Access Separations and Control on the Crossroad at Interchanges 753 

Safety 755 

Stage Development 755 

Economic Factors 755 

Initial Costs 755 

Maintenance Costs 755 

Vehicular Operating Costs 756 

Grade Separation Structures.. 756 

Introduction 756 

Types of Separation Structures 756 

Overpass Versus Underpass Roadways.... 762 

General Design Considerations 762 

Structure Widths 764 

Underpass Roadways 765 

Lateral Clearances 765 

Vertical Clearance 767 

Overpass Roadways 768 

Bridge Railings 768 

Lateral Clearances 770 

Medians 770 

Longitudinal Distance to Attain Grade Separation 771 

Grade Separations Without Ramps 773 

Interchanges 774 

General Considerations 774 

Three-Leg Designs. 775 

Four-Leg Designs 780 

Ramps in One Quadrant 780 

Diamond Interchanges 782 

Single-Point Urban Interchanges 787 

Cloverleafs 792 

Directional and Semidirectional Interchanges 798 

Other Interchange Configurations 803 

Offset Interchanges 803 

Combination Interchanges 803 

General Design Considerations 806 

xxvii 



Determination of Interchange Configuration 806 

Approaches to the Structure 809 

Interchange Spacing 811 

Uniforniity of Interchange Patterns 811 

Route Continuity 811 

Overlapping Routes 813 

Signing and Marking..... 813 

Basic Number of Lanes 814 

Coordination of Lane Balance and Basic Number of Lanes 815 

Auxiliary Lanes 818 

Lane Reductions 822 

Weaving Sections 823 

Collector-Distributor Roads 823 

Tw^o-Exit Versus Single-Exit Interchange Design 824 

Wrong- Way Entrances 825 

Ramps ..827 

Types and Examples 827 

General Ramp Design Considerations 829 

Ramp Traveled-Way Widths 842 

Ramp Terminals 844 

Single-Lane Free-Flow Terminals, Entrances 849 

Single-Lane Free-Flow Terminals, Exits 853 

Other Interchange Design Features 867 

Testing for Ease of Operation 867 

Pedestrians 868 

Ramp Metering 869 

Grading and Landscape Development 869 

Models 871 

References 871 



XXVlll 



LIST OF EXHIBITS 

Page 

1-1 Hierarchy of Movement 2 

1-2 Channelization of Trips 4 

1-3 Schematic Illustration of a Functionally Classified Rural Highway Network 5 

1-4 Schematic Illustration of a Portion of a Suburban Street Network 6 

1-5 Relationship of Functionally Classified Systems in Serving Traffic Mobility and 

Land Access 7 

1-6 Typical Distribution of Rural Functional Systems 10 

1-7 Typical Distribution of Urban Functional Systems 12 

2-1 Design Vehicle Dimensions 16 

2-2 Minimum Turning Radii of Design Vehicles 19 

2-3 Minimum Turning Path for Passenger Car (P) Design Vehicle 21 

2-4 Minimum Turning Path for Single-Unit (SU) Truck Design Vehicle 22 

2-5 Minimum Turning Path for hitercity Bus (BUS-12 [BUS-40]) Design Vehicle 23 

2-6 Minimum Turning Path for Intercity Bus (BUS-14 [BUS-45]) Design Vehicle 24 

2-7 Minimum Turning Path for City Transit Bus (CITY-BUS) Design Vehicle 25 

2-8 Minimum Turning Path for Conventional School Bus (S-BUS-1 1 [S-BUS-36]) 

Design Vehicle 26 

2-9 Minimum Turning Path for Large School Bus (S-BUS-12 [S-BUS-40]) 

Design Vehicle 27 

2-10 Minimum Turning Path for Articulated Bus (A-BUS) Design Vehicle. 28 

2-1 1 Turning Characteristics of a Typical Tractor-Semitrailer Combination Truck 29 

2-12 Lengths of Commonly Used Truck Tractors 30 

2-13 Minimum Turning Path for hitermediate Semitrailer (WB-12 [WB-40]) 

Design Vehicle 31 

2-14 Minimum Turning Path for hitermediate Semitrailer (WB-15 [WB-50]) 

Design Vehicle 32 

2-15 Minimum Turning Path for hiterstate Semitrailer (WB-19 [WB-62]) 

Design Vehicle 33 

2-16 Minimum Turning Path for Interstate Semitrailer (WB-20 [WB-65 and WB-67]) 

Design Vehicle 34 

2-17 Minimum Turning Path for Double-Trailer Combination (WB-20D [WB-67D]) 

Design Vehicle 35 

2-18 Minimum Turning Path for Triple-Trailer Combination (WB-30T [WB-IOOT]) 

Design Vehicle 36 

2-19 Minimum Turning Path for Turnpike-Double Combination (WB-33D [WB-109D]) 

Design Vehicle 37 

2-20 Minimum Turning Path for Motor Home (MH) Design Vehicle 38 

2-21 Minimum Turning Path for Passenger Car and Camper Trailer (P/T) 

Design Vehicle 39 

2-22 Minimum Turning Path for Passenger Car and Boat Trailer (P/B) Design Vehicle 40 

2-23 Minimum Turning Path for Motor Home and Boat Trailer (MH/B) Design Vehicle ... 41 
2-24 Acceleration of Passenger Cars, Level Conditions 44 



2-25 Deceleration Distances for Passenger Vehicles Approaching Intersections 45 

2-26 Median Driver Reaction Time to Expected and Unexpected Information 51 

2-27 85th-Percentile Driver Reaction Time to Expected and Unexpected Information... 52 

2-28 Relation Between Peak-Hour and Average Daily Traffic Volumes on 

Rural Arterials 60 

2-29 Corresponding Design Speeds in Metric and US Customary Units 70 

2-30 Generalized Speed-Volume-Density Relationships (15) 73 

2-31 General Definitions of Levels of Service 84 

2-32 Guidelines for Selection of Design Levels of Service 85 

2-33 Weaving Sections 87 

2-34 Simple and Multiple Weaving Sections 87 

2-35 Estimated Crash Rates by Type of Median — Urban and Suburban Areas (18) 93 

2-36 Estimated Crash Rates by Type of Median — Rural Areas (18) 94 

2-37 Estimated Crash Rates by Unsignalized and Signalized Access Density — 

Urban and Suburban Areas (18) 95 

3-1 Stopping Sight Distance 112 

3-2 Stopping Sight Distance on Grades 115 

3-3 Decision Sight Distance 116 

3-4 Elements of Passing Sight Distance for Two-Lane Highways 119 

3-5 Elements of Safe Passing Sight Distance for Design of Two-Lane Highways 120 

3-6 Total Passing Sight Distance and Its Components — Two-Lane Highways 123 

3-7 Passing Sight Distance for Design of Two-Lane Highways 124 

3-8 Scaling and Recording Sight Distances on Plans 129 

3-9 Geometry for Ball-Bank Indicator 134 

3-10 Side Friction Factors 136 

3-11 Comparison of Side Friction Factors Assumed for Design of Different Types of 

Facilities 137 

3-12 Methods of Distributing Superelevation and Side Friction 139 

3-13 Side Friction Factors for Rural Highways and High-Speed Urban Streets 144 

3-14 Minimum Radius for Design of Rural Highways, Urban Freeways, 

and High-Speed Urban Streets Using Limiting Values of e and f. 145 

3-15 Method 5 Procedure for Development of the Finalized e Distribution 146 

3-16 Design Superelevation Rates for Maximum Superelevation Rate of 4 Percent 147 

3-17 Design Superelevation Rates for Maximum Superelevation Rate of 6 Percent 148 

3-18 Design Superelevation Rates for Maximum Superelevation Rate of 8 Percent 149 

3-19 Design Superelevation Rates for Maximum Superelevation Rate of 10 Percent 150 

3-20 Design Superelevation Rates for Maximum Superelevation Rate of 12 Percent 151 

3-21 Values for Design Elements Related to Design Speed and Horizontal Curvature 156 

3-22 Values for Design Elements Related to Design Speed and Horizontal Curvature 158 

3-23 Values for Design Elements Related to Design Speed and Horizontal Curvature 160 

3-24 Values for Design Elements Related to Design Speed and Horizontal Curvature 162 

3-25 Values for Design Elements Related to Design Speed and Horizontal Curvature 164 

3-26 Minimum Curve Radius for Section with Normal Cross Slopes (emax = 10%) 168 

3-27 Maximum Relative Gradients 170 

3-28 Adjustment Factor for Number of Lanes Rotated 172 

3-29 Minimum Superelevation Runoff and Tangent Runout Lengths 174 



3-30 Runoff Locations that Minimize the Vehicle's Lateral Motion 175 

3-31 Limiting Superelevation Rates 176 

3-32 Transition Spirals (23) 178 

3-33 Maximum Radius for Use of a Spiral Curve Transition 179 

3-34 Desirable Length of Spiral Curve Transition.: 181 

3-35 Superelevation Rates Associated With Large Relative Gradients 182 

3-36 Tangent Runout Length for Spiral Curve Transition Design 183 

3-37 Diagrammatic Profiles Showing Methods of Attaining Superelevation for a 

Curve to the Right 185 

3-38 Lengths of Circular Arcs for Different Compound Curve Radii 192 

3-39 Side Friction Factors for Low-Speed Urban Streets 194 

3-40 Relationship of Radius Superelevation, Cross Slope Rate, and Design Speed for 

Low-Speed Urban Street Design 196 

3-41 Minimum Radii and Minimum Lengths of Superelevation Runoff for Limiting 

Values of e and f (Low-Speed Urban Streets) 197 

3-42 Relation Between Speed and Side Friction Factor on Curves at Intersections 200 

3-43 Minimum Radii for hitersection Curves 201 

3-44 Minimum Radii for Curves at Intersections 202 

3-45 Minimum Lengths of Spiral for Intersection Curves 204 

3-46 Length of Circular Arc for a Compound Intersection Curve When Followed by 

a Curve of One-Half Radius or Preceded by a Curve of Double Radius 206 

3-47 Track Width for Widening of Traveled Way on Curves 208 

3-48 Front Overhang for Widening of Traveled Way on Curves 210 

3-49 Extra Width Allowance for Difficulty of Driving on Traveled Way on Curves 211 

3-50 Widening Components on Open Highway Curves (Two-Lane Highways, 

One-Way or Two- Way) 213 

3-5 1 Calculated and Design Values for Traveled Way Widening on Open Highway 

Curves (Two-Lane Highways, One-Way or Two-Way) 215 

3-52 Adjustments for Traveled Way Widening Values on Open Highway Curves 

(Two-Lane Highways, One-Way or Two-Way) 217 

3-53 Derivation of Tuming Roadway Widths on Curves at Intersections 219 

3-54 Derived Pavement Widths for Tuming Roadways for Different Design Vehicles 221 

3-55 Design Widths of Pavements for Tuming Roadways 224 

3-56 Range of Usable Shoulder Widths or Equivalent Lateral Clearances Outside of 

Tuming Roadways, Not on Structure 228 

3-57 Design Controls for Stopping Sight Distance on Horizontal Curves 229 

3-58 Diagram Illustrating Components for Determining Horizontal Sight Distance 23 1 

3-59 Speed-Distance Curves for a Typical Heavy Truck of 120 kg/kW [200 Ib/hp] for 

Deceleration on Upgrades 237 

3-60 Speed-Distance Curves for Acceleration of a Typical Heavy Truck of 120 kg/kW 

[200 Ib/hp] on Upgrades and Downgrades 238 

3-61 Speed-Distance Curves for a Typical Recreational Vehicle on the Selected 

Upgrades (40) 240 

3-62 Crash Involvement Rate of Trucks for Which Running Speeds Are Reduced 

Below Average Running Speed of All Traffic (41) 241 



./VvV^t 



3-63 Critical Lengths of Grade for Design, Assumed Typical Heavy Truck of 

120 kg/kW [200 Ib/hp], Entering Speed = 1 10 km/h [70 mph] 245 

3-64 Critical Lengths of Grade Using an Approach Speed of 90 km/h [55 mph] for 

Typical Recreational Vehicle (40) 246 

3-65 Climbing Lanes on Two-Lane Highways 248 

3-66 Climbing Lane on Freeways and Multilane Highways 254 

3-67 Passing Lanes Section on Two-Lane Roads 256 

3-68 Recommended Lengths of Turnouts Including Taper 258 

3-69 Forces Acting on a Vehicle in Motion 260 

3-70 Rolling Resistance of Roadway Surfacing Materials 260 

3-71 Basic Types of Emergency Escape Ramps 263 

3-72 Typical Emergency Escape Ramp 267 

3-73 Types of Vertical Curves 269 

3-74 Parameters Considered in Determining the Length of a Crest Vertical Curve to 

Provide Sight Distance 271 

3-75 Design Controls for Crest Vertical Curves — Open Road Conditions 273 

3-76 Design Controls for Stopping Sight Distance and for Crest and Sag 

Vertical Curves 274 

3-77 Design Controls for Crest Vertical Curves Based on Passing Sight Distance 276 

3-78 Design Controls for Sag Vertical Curves — Open Road Conditions 278 

3-79 Design Controls for Sag Vertical Curves 280 

3-80 Sight Distance at Undercrossings 281 

3-81 Alignment and Profile Relationships in Roadway Design (48) 287 

4-1 Typical Cross Section, Normal Crown 310 

4-2 Typical Cross Section, Superelevated 311 

4-3 Roadway Sections for Divided Highway (Basic Cross Slope Arrangements) 312 

4-4 Normal Traveled-Way Cross Slope 314 

4-5 Graded and Usable Shoulders 317 

4-6 Typical Highway Curbs 325 

4-7 Designation of Roadside Regions 330 

4-8 Typical Frontage Road Arrangements 344 

4-9 Frontage Roads, Irregular Pattern ,.... 345 

4-10 One-way Frontage Roads, Entrance and Exit Ramps 346 

4-11 Two-way Frontage Roads, Entrance and Exit Ramps 346 

4-12 Frontage Road in Business Area With Narrow Outer Separation 347 

4-13 Typical Outer Separations 348 

4-14 Noise-Abatement Criteria for Various Land Uses 350 

4-15 Effects of Depressing the Highway 351 

4-16 Effects of Elevating the Highway 352 

4-17 Typical Two-lane Tunnel Sections 358 

4-18 Diagrammatic Tunnel Sections 360 

4-19 Entrance to a Freeway Tunnel 360 

4-20 Interior of a 3-lane One-way Tunnel 361 

4-21 Typical Pedestrian Overpasses on Major Highways 366 

4-22 Mid-block Sidewalk Curb Ramp Details 368 



.ArtX'^LL 



4-23 Sidewalk Curb Ramp at Middle of Radius — Discouraged Where Pedestrian 

and/or Vehicular Volumes are Moderate to High 369 

4-24 Sidewalk Curb Ramp at End of Curb Radius 370 

4-25 Sidewalk Curb Ramp at Midblock 370 

4-26 Median and Island Openings 371 

4-27 Bus Turnouts 373 

4-28 Midblock Bus Turnout 374 

4-29 Sawtooth Bus Loading Area 376 

4-30 Typical Park-and-Ride Facility 378 

4-31 Parking Lane Transition at Intersection 379 

5-1 Minimum Design Speeds for Local Rural Roads 385 

5-2 Design Controls for Stopping Sight Distance and for Crest and Sag Vertical 

Curves 385 

5-3 Design Controls for Crest Vertical Curves Based on Passing Sight Distance 386 

5-4 Maximum Grades for Local Rural Roads 386 

5-5 Minimum Width of Traveled Way and Shoulders 388 

5-6 Minimum Clear Roadway Widths and Design Loadings for New and 

Reconstructed Bridges 390 

5-7 Minimum Structural Capacities and Minimum Roadway Widths for Bridges to 

Remain in Place 390 

5-8 Types of Cul-de-Sacs and Dead-End Streets 399 

5-9 Alley Turnarounds..... 401 

5-10 Actual Curb Radius and Effective Radius for Right-Turn Movements at 

Intersections 405 

5-11 Minimum Illumination Levels 406 

5-12 Potential Road Network 409 

5-13 Design Controls for Stopping Sight Distance and for Crest and Sag Vertical 

Curves — Recreational Roads 411 

5-14 Design Controls for Passing Sight Distance for Crest Vertical Curves — 

Recreational Roads 412 

5-15 Maximum Grades for Recreational Roads 413 

5-16 Minimum-Radius Horizontal Curve for Gravel Surface 414 

5-17 Turnout Design 416 

5-18 Widths of Traveled Way and Shoulders — Recreational Roads 416 

5-19 Design Speeds for Resource Recovery and Local Service Roads 419 

6-1 Minimum Design Speeds for Rural Collectors 426 

6-2 Design Controls for Stopping Sight Distance and for Crest and Sag Vertical 

Curves 426 

6-3 Design Controls for Crest Vertical Curves Based on Passing Sight Distance 427 

6-4 Maximum Grades for Rural Collectors 427 

6-5 Minimum Width of Traveled Way and Shoulders 429 

6-6 Minimum Roadway Widths and Design Loadings for New and Reconstructed 

Bridges 430 

6-7 Structural Capacities and Minimum Roadway Widths for Bridges to Remain in 

Place 431 

6-8 Maximum Grades for Urban Collectors 436 



7-1 Minimum Sight Distances for Arterials 449 

7-2 Maximum Grades for Rural Arterials 450 

7-3 Minimum Width of Traveled Way and Usable Shoulder for Rural Arterials 452 

7-4 Climbing Lane on Two-Lane Rural Arterial 454 

7-5 Two-Lane Arterial Cross Section With Ultimate Development to a Four-Lane 

Arterial .457 

7-6 Methods of Attaining Superelevation on Divided Arterials 465 

7-7 Typical Medians on Divided Arterials 466 

7-8 Cross Sectional Arrangements on Divided Arterials 468 

7-9 Cross Sectional Arrangements on Divided Arterials 469 

7-10 Maximum Grades for Urban Arterials 476 

7-1 1 Continuous Two-Way Left-Turn Lane 480 

7-12 Parking Turnouts in Downtown District 483 

7-13 Arterial Street in Residential Area 484 

7-14 Divided Arterial Street With Parking Lanes 485 

7-15 Urban Arterial With Dual Left-Turn Lanes 494 

7-16 Divided Arterial Street With Two-Way Frontage Road 499 

7-17 Bus Stops at Special Locations Adjacent to Certain Arterials 503 

7-18 Exclusive Bus Lane 505 

8-1 Maximum Grades for Rural and Urban Freeways 510 

8-2 Typical Ground-Level Rural Freeway 514 

8-3 Typical Rural Medians 515 

8-4 Typical Cross Section for Depressed Freeways 520 

8-5 Restricted Cross Sections for Depressed Freeways 520 

8-6 Cross Sections with Retaining Walls on Depressed Freeways Without Ramps 522 

8-7 Depressed Freeway 523 

8-8 Depressed Freeway 524 

8-9 Typical Cross Sections for Elevated Freeways on Structures Without Ramps 528 

8-10 Typical and Restricted Cross Sections for Elevated Freeways on Structure With 

Frontage Roads 529 

8-1 1 Typical and Restricted Cross Sections for Elevated Freeways on Embankment 530 

8-12 Viaduct Freeway... 531 

8-13 Two-Level Viaduct Freeway 532 

8-14 Typical Cross Sections for Ground-Level Freeways 533 

8-15 Restricted Cross Sections for Ground-Level Freeways 534 

8-16 Profile Control — Rolling Terrain Combination-Type Freeway 535 

8-17 Profile Control — Flat Terrain Combination-Type Freeway 536 

8-18 Cross-Section Control — Combination-Type Freeway 538 

8-19 Combination-Type Freeway 539 

8-20 Four-Level Cantilevered Freeway 540 

8-21 Typical Cross Sections for Reverse-Flow Operation 541 

8-22 Typical Reverse Roadway Tenninals 543 

8-23 Reverse-Flow Freeway 544 

8-24 Typical Dual-Divided Freeway 546 

8-25 Dual-Divided Freeway With a 4-3-3-4 Roadway Arrangement 546 

8-26 Bus Roadway Located Between a Freeway and a Parallel Frontage Road 548 

xxxiv 



8-27 Bus Stops at Freeway Level 551 

8-28 Bus Stops at Freeway-Level Diamond Interchange 552 

8-29 Freeway-Level Bus Stop at Cloverleaf Interchange 552 

8-30 Bus Stops at Street Level on Diamond Interchange 553 

8-31 Joint Freeway-Transit Right-of-Way 555 

8-32 Typical Sections With Rail Transit in Freeway Median 556 

8-33 Example of Transit Station Layout 557 

8-34 Depressed Freeway With Rail Rapid Transit in the Median 558 

9-1 Physical and Functional Intersection Area 561 

9-2 Elements of the Functional Area of an Intersection 561 

9-3 Channelized High-Type "T" Intersections 563 

9-4 Three-Leg Rural Intersection, Channelized "T" 563 

9-5 "T" Intersections *. 564 

9-6 Channelized 'T' Intersections 565 

9-7 "T" Intersections 566 

9-8 Channelized 'T" hitersections 567 

9-9 Unchannelized Four-Leg Intersections, Plain and Flared 570 

9-10 Channelized Four-Leg Intersections 571 

9-1 1 Channelized Four-Leg Intersections 572 

9-12 Four-Leg Intersections (Channelized High-type) 574 

9-13 Four-Leg Intersections (Channelized High-type) 576 

9-14 Realigning Multi-Leg Intersections 577 

9-15 Geometric Elements of a Single-Lane Modem Roundabout 579 

9-16 Typical Modem Roundabout 580 

9-17 Roundabout with Entry Flaring in Two Quadrants 582 

9-18 Realignment Variations at Intersections 584 

9-19 Edge-of-Traveled-Way Designs for Turns at Intersections 588 

9-20 Edge of Traveled Way for Turns at Intersections 592 

9-21 Minimum Traveled Way (Passenger Vehicles) 598 

9-22 Minimum Traveled Way Designs (Single-Unit Trucks and City Transit Buses) 600 

9-23 Minimum Edge-of-Traveled-Way Designs (WB-12 [WB-40] Design 

Vehicle Path).... 602 

9-24 Minimum Edge-of-Traveled-Way Designs (WB - 1 5 [ WB-50] ) Design 

Vehicle Path) 604 

9-25 Minimum Edge-of-Traveled-Way Designs (WB-15 [WB-50]) Design 

Vehicle Path) 605 

9-26 Minimum Edge-of-Traveled-Way Designs (WB-19 [WB-62]) Design 

Vehicle Path) 607 

9-27 Minimum Edge-of-Traveled-Way Designs (WB-30T [WB-IOOT] Design 

Vehicle Path) 609 

9-28 Minimum Edge-of-Traveled-Way Designs (WB-33D [WB-109D] Design 

Vehicle Path) 611 

9-29 Effect of Curbed Radii on Right Turning Paths of Various Design Vehicles 616 

9-30 Effect of Curbed Radii on Right Tuming Paths of Various Design Vehicles ....617 

9-3 1 Cross Street Width Occupied by Tuming Vehicle for Various Angles of 

Intersection and Curb Radii 619 

XXXV 



9-32 Effect of Curbed Radii and Parking on Right Turning Paths 621 

9-33 Variations in Length of Crosswalk With Different Curb Radii and Width of 

Borders 624 

9-34 Comer Setbacks with Different Curb Radii and Width of Borders 624 

9-35 General Types and Shapes of Islands and Medians 628 

9-36 Alignment for Addition of Divisional Islands at Intersections 630 

9-37 Details of Comer Island Designs for Turning Roadways (Urban Location) 634 

9-38 Details of Comer Island Designs for Turning Roadways (Rural Cross Section on 

Approach) 635 

9-39 Nose Ramping at Approach End of Median or Comer Island 636 

9-40 Details of Divisional Island Design.. 637 

9-41 Minimum Turning Roadway Designs With Comer Islands at Urban Locations ........ 639 

9-42 Typical Designs for Tuming Roadways , 642 

9-43 Use of Simple and Compound Curves at Free Flow Tuming Roadways 644 

9-44 Effective Maximum Relative Gradients 647 

9-45 Development of Superelevation at Tuming Roadway Terminals 648 

9-46 Development of Superelevation at Tuming Roadway Terminals 649 

9-47 Development of Superelevation at Tuming Roadway Terminals 650 

9-48 Development of Superelevation at Tuming Roadway Terminals 651 

9-49 Maximum Algebraic Difference in Cross Slope at Tuming Roadway Terminals ...... 652 

9-50 Intersection Sight Triangles 656 

9-51 Length of Sight Triangle Leg— Case A— No Traffic Control 659 

9-52 Length of Sight Triangle Leg— Case A— No Traffic Control 661 

9-53 Adjustment Factors for Sight Distance Based on Approach Grade 662 

9-54 Time Gap for Case B 1— Left Turn from Stop 664 

9-55 Design Intersection Sight Distance — Case Bl — Left Turn From Stop 665 

9-56 Intersection Sight Distance— Case Bl— Left Turn from Stop 666 

9-57 Time Gap for Case B2 — Right Turn from Stop and Case B3 — Crossing Maneuver.. 668 

9-58 Design Intersection Sight Distance — Case B2 — Right Tum from Stop and 

Case B 3— Crossing Maneuver 668 

9-59 Intersection Sight Distance— Case B2 — Right Tum from Stop and Case B3— - 

Crossing Maneuver 669 

9-60 Case CI — Crossing Maneuvers From Yield-Controlled Approaches — ^Length of 

Minor Road Leg and Travel Times 672 

9-61 Length of Sight Triangle Leg Along Major Road — Case CI — Crossing 

Maneuver at Yield Controlled Intersections 673 

9-62 Length of Sight Triangle Leg Along Major Road for Passenger Cars — 

Case CI — Crossing Maneuver 674 

9-63 Time Gap for Case C2— Left or Right Tum 676 

9-64 Design Intersection Sight Distance — Case C2— Left or Right Tum at 

Yield Controlled Intersections 676 

9-65 Intersection Sight Distance — Case C2— Yield Controlled Left or Right Tum 677 

9-66 Time Gap for Case F— Left Turns From the Major Road 678 

9-67 Intersection Sight Distance — Case F — Left Tum From Major Road 679 

9-68 Intersection Sight Distance — Case F — Left Tum From Major Road 680 

9-69 Sight Triangles at Skewed Intersections 681 

xxxvi 



9-70 Stopping Sight Distance for Turning Roadways 682 

9-71 Two-Lane Crossroad Designs to Discourage Wrong-Way Entry 684 

9-72 Divided Crossroad Designs to Discourage Wrong- Way Entry 685 

9-73 General Types of Intersections 687 

9-74 General Types of Intersections 688 

9-75 Guide for Left-Turn Lanes on Two-Lane Highways (6) 689 

9-76 Control Radii at Intersections for 90-Degree Left Turns 695 

9-77 Minimum Design of Median Openings (P Design Vehicle, Control Radius of 

12 m [40 ft]) 697 

9-78 Minimum Design of Median Openings (P Design Vehicle, Control Radius of 

12 m [40 ft]) 698 

9-79 Minimum Design of Median Openings (SU Design Vehicle, Control Radius of 

15 m [50 ft]) 698 

9-80 Minimum Design of Median Openings (WB-12 [WB-40] Design Vehicle, 

Control Radius of 23 m [75 ft]) 699 

9-81 Minimum Design of Median Openings (SU Design Vehicle, Control Radius 

of 15 m [50 ft]) 699 

9-82 Minimum Design of Median Openings (WB-12 [WB-40] 

Design Vehicle, Control Radius of 23 m [75 ft]) 700 

9-83 Minimum Design of Median Openings (Radius of 30 m [100 ft]). 700 

9-84 Minimum Design of Median Openings (Effect of Skew) 704 

9-85 Design Controls for Minimum Median Openings 705 

9-86 Effect of Skew on Minimum Design for Median Openings (Typical Values 

Based on Control Radius of 15 m [50 ft]) 707 

9-87 Above Minimum Design of Median Openings (Typical Bullet-Nose Ends) 708 

9-88 Jughandle-Type Ramp with Crossroad 710 

9-89 At-Grade Loop (Surface Loop) with Crossroad 710 

9-90 Special Indirect Left-Turn Designs for Traffic Leaving Highway with 

Narrow Median 711 

9-91 Indirect Left Turn Through a Crossover 713 

9-92 Minimum Designs for U-turns 715 

9-93 Special Indirect U-Tum with Narrow Medians 716 

9-94 Flush or Traversable Median Lane Markings 717 

9-95 Taper Design for Auxiliary Lanes (Metric) 721 

9-96 4.2 to 5.4 m [14 to 18 ft] Median Width Left-Turn Design (Metric) 723 

9-97 Median Left-Turn Design for Median Width in Excess of 5.4 m [18 ft] 725 

9-98 Parallel and Tapered Offset Left-Turn Lane 728 

9-99 Four-Leg Intersection Providing Simultaneous Left Turns... 729 

9-100 Intersections with Frontage Roads 731 

9-101 Cumulative Frequency Distribution of Impact Lengths 734 

9-102 Railroad-Highway Grade Crossing 736 

9-103 Case A: Moving Vehicle to Safely Cross or Stop at Railroad Crossing 739 

9-104 Required Design Sight Distance for Combination of Highway and Train Vehicle 

Speeds; 20-m [65-ft] Truck Crossing a Single Set of Tracks at 90° 741 

9-105 Case B: Departure of Vehicle From Stopped Position to 

Cross Single Railroad Track 742 

xxxvii 



10-1 Interchange Configurations 748 

10-2 Factors Influencing Length of Access Control Along an Interchange Crossroad 754 

10-3 Typical Grade Separation Structures With Closed Abutments 760 

10-4 Typical Grade Separation Structure With Open-End Span 761 

10-5 Multilevel Grade Separation Structures 762 

10-6 Lateral Clearances for Major Roadway Underpasses .,,.. 766 

10-7 Typical Overpass Structures 769 

10-8 Flat Terrain, Distance Required to Effect Grade Separation 772 

10-9 Three-Leg Interchanges With Single Structures 776 

10-10 Three-Leg Interchanges With Multiple Structures 777 

10-1 1 Three-Leg Interchange (T-Type or Trumpet) ,... 778 

10-12 Three-Leg Interchange Semidirectional Design 778 

10-13 Directional Three-Leg Interchange of a River Crossing 779 

10-14 Trumpet Freeway -to-Freeway Interchange 780 

10-15 Four-Leg Interchanges, Ramps in One Quadrant 781 

10-16 Diamond Interchanges, Conventional Arrangements 783 

10-17 Diamond Interchange Arrangements to Reduce Traffic Conflicts 783 

10-18 Diamond Interchanges with Additional Structures 784 

10-19 Freeway With a Three-Level Diamond Interchange 785 

10-20 Existing Four-Leg Interchange With Diamond Stage Construction 786 

10-21 X-Pattem Ramp Arrangement 786 

10-22 Underpass Single Point Urban Interchange 788 

10-23 An SPUI Underpass in Restricted Right-of-Way 788 

10-24 Overpass Layout With a Frontage Road and a Separate U-Tum Movement 790 

10-25 Underpass SPUI and Overpass SPUI .....791 

10-26 Four-Leg Interchange, Full Cloverleaf With Collector-Distributor Roads 793 

10-27 Cloverleaf Interchange With Collector-Distributor Roads 794 

10-28 Schematic of Partial Cloverleaf Ramp Arrangements, Exit and Entrance Turns 795 

10-29 Four-Leg Interchange (Partial or Two-Quadrant Cloverleaf with Ramps Before 

Main Structure) 796 

10-30 Four-Leg Interchange (Partial or Two-Quadrant Cloverleaf with Ramps Beyond 

Main Structure) 797 

10-31 Semidirect Interchanges With Weaving 799 

10-32 Semidirect Interchanges With No Weaving 799 

1 0-3 3 Semidirectional and Directional Interchanges — Multilevel Structures 800 

10-34 Directional Interchange, Two Semidirect Connections 801 

10-35 Four-Level Directional Interchange 801 

10-36 Four-Level Directional Interchange 802 

10-37 Semidirectional Interchange With Loops 802 

10-38 Offset Interchange via Ramp Highway 803 

1 0-39 Four-Leg Interchange, Diamond With a Semidirect Connection 804 

10-40 Four-Leg Interchange, Cloverleaf With a Semidirect Connection 805 

10-41 Complex Interchange Arrangement 805 

10-42 Freeway with a Three-Level Cloverleaf Interchange 806 

10-43 Adaptability of Interchanges on Freeways as Related to Types of Intersecting 

Facilities 808 

xxxviii 



10-44 Widening for Divisional Island at Interchanges 810 

10-45 Arrangement of Exits Between Successive Interchanges 812 

10-46 Interchange Forms to Maintain Route Continuity 812 

10-47 Collector-Distributor Road on Major-Minor Roadway Overlap 814 

10-48 Schematic of Basic Number of Lanes 815 

10-49 Typical Examples of Lane Balance 816 

10-50 Coordination of Lane Balance and Basic Number of Lanes 817 

10-51 Alternative Methods of Dropping Auxiliary Lanes 819 

10-52 Coordination of Lane Balance and Basic Number of Lanes Through Application of 

Auxiliary Lanes 820 

10-53 Auxiliary Lane Dropped at Two-Lane Exit 821 

10-54 Interchange Forms with One and Two Exits 826 

10-55 General Types of Ramps 828 

10-56 Guide Values for Ramp Design Speed as Related to Highway Design Speed 830 

10-57 Ramp Shapes..... 831 

10-58 Development of Superelevation at Free-Flow Ramp Terminals 835 

10-59 Typical Gore Area Characteristics 837 

10-60 Typical Gore Details 838 

10-61 Minimum Length of Taper Beyond an Offset Nose 839 

10-62 Traveled-Way Narrowing on Entrance Ramps 839 

10-63 Gore Area, Single-Lane Exit 840 

10-64 Gore Area, Major Fork 840 

10-65 Gore Area, Two-Lane Exit 841 

10-66 Entrance Terminal 841 

10-67 Design Widths for Turning Roadways 843 

10-68 Recommended Minimum Ramp Terminal Spacing 848 

10-69 Typical Single-Lane Entrance Ramps 849 

10-70 Minimum Acceleration Lengths for Entrance Terminals With Flat Grades of 

2 Percent or Less 851 

10-71 Speed Change Lane Adjustment Factors as a Function of Grade 852 

10-72 Exit Ramps— Single Lane 854 

10-73 Minimum Deceleration Lengths for Exit Terminals With Flat Grades of 

2 Percent or Less 855 

10-74 Layout of Taper-Type Terminals on Curves (Metric) 857 

10-75 Parallel-Type Ramp Terminals on Curves 859 

10-76 Typical Two-Lane Entrance Ramps 862 

10-77 Two-Lane Exit Terminals 863 

10-78 Major Forks 865 

10-79 Branch Connections 866 

10-80 Diagram of Freeway Operational Problem and Solution 868 



^JvJ\tL,^ 



Foreword 



As highway designers, highway engineers strive to provide for the needs of highway users 
while maintaining the integrity of the environment. Unique combinations of design requirements 
that are often conflicting result in unique solutions to the design problems. The guidance supplied 
by this text, A Policy on Geometric Design of Highways and Streets, is based on established 
practices and is supplemented by recent research. This text is also intended to form a 
comprehensive reference manual for assistance in administrative, planning, and educational 
efforts pertaining to design formulation. 

Design values are presented in this document in both metric and U.S. customary units and 
were developed independently within each system. The relationship between the metric and U.S. 
customary values is neither an exact (soft) conversion nor a completely rationalized (hard) 
conversion. The metric values are those that would have been used had the policy been presented 
exclusively in metric units; the U.S. customary values are those that would have been used if the 
policy had been presented exclusively in U.S. customary units. Therefore, the user is advised to 
work entirely in one system and not attempt to convert directly between the two. 

The fact that new design values are presented herein does not imply that existing streets and 
highways are unsafe, nor does it mandate the initiation of improvement projects. This publication 
is not intended as a policy for resurfacing, restoration, or rehabilitation (3R) projects. For projects 
of this type, where major revisions to horizontal or vertical curvature are not necessary or 
practical, existing design values may be retained. Specific site investigations and crash history 
analysis often indicate that the existing design features are performing in a satisfactory manner. 
The cost of full reconstruction for these facilities, particularly where major realignment is not 
needed, will often not be justified. Resurfacing, restoration, and rehabilitation projects enable 
highway agencies to improve highway safety by selectively upgrading existing highway and 
roadside features without the cost of full reconstruction. When designing 3R projects, the 
designer should refer to TRB Special Report 214, Designing Safer Roads: Practices for 
Resurfacing, Restoration, and Rehabilitation and related pubUcations for guidance. 

The intent of this policy is to provide guidance to the designer by referencing a 
recommended range of values for critical dimensions. It is not intended to be a detailed design 
manual that could supercede the need for the application of sound principles by the 
knowledgeable design professional. Sufficient flexibility is permitted to encourage independent 
designs tailored to particular situations. Minimum values are either given or implied by the lower 
value in a given range of values. The larger values within the ranges will normally be used where 
the social, economic, and environmental (S.E.E.) impacts are not critical. 

The highway, vehicle, and individual users are all integral parts of transportation safety and 
efficiency. While this document primarily addresses geometric design issues, a properly equipped 
and maintained vehicle and reasonable and prudent performance by the user are also necessary 
for safe and efficient operation of the transportation facility. 



xli 



Emphasis has been placed on the joint use of transportation corridors by pedestrians, 
cyclists, and public transit vehicles. Designers should recognize the implications of this sharing of 
the transportation corridors and are encouraged to consider not only vehicular movement, but also 
movement of people, distribution of goods, and provision of essential services. A more 
comprehensive transportation program is thereby emphasized. 

Cost-effective design is also emphasized. The traditional procedure of comparing highway- 
user benefits with costs has been expanded to reflect the needs of non-users and the environment. 
Although adding complexity to the analysis, this broader approach also takes into account both 
the need for a given project and the relative priorities among various projects. The results of this 
approach may need to be modified to meet the needs-versus-funds problems that highway 
administrators face. The goal of cost-effective design is not merely to give priority to the most 
beneficial individual projects but to provide the most benefits to the highway system of which 
each project is a part. 

Most of the technical material that follows is detailed or descriptive design information. 
Design guidelines are included for freeways, arterials, collectors, and local roads, in both urban 
and rural locations, paralleling the functional classification used in highway planning. The book 
is organized into functional chapters to stress the relationship between highway design and 
highway function. An explanation of functional classification is included in Chapter 1. 

These guidelines are intended to provide operation efficiency, comfort, safety, and 
convenience for the motorist. The design concepts presented herein were also developed with 
consideration for environmental quality. The effects of the various environmental impacts can 
and should be mitigated by thoughtful design processes. This principle, coupled with that of 
aesthetic consistency with the surrounding terrain and urban setting, is intended to produce 
highways that are safe and efficient for users, acceptable to non-users, and in harmony with the 
environment. 

This publication supersedes the 1994 AASHTO publication of the same name. Because the 
concepts presented could not be completely covered in one book, references to additional 
literature are given at the end of each chapter. 



xlii 



CHAPTER 1 
HIGHWAY FUNCTIONS 

SYSTEMS AND CLASSIFICATIONS 

The classification of highways into different operational systems, functional classes, or 
geometric types is necessary for communication among engineers, administrators, and the general 
public. Different classification schemes have been applied for different purposes in different rural 
and urban regions. Classification of highways by design types based on the major geometric 
features (e.g., freeways and conventional streets and highways) is the most helpful one for 
highway location and design procedures. Classification by route numbering (e.g., U.S., State, 
County) is the most helpful for traffic operations. Administrative classification (e.g.. National 
Highway System or Non-National Highway System) is used to denote the levels of government 
responsible for, and the method of financing, highway facilities. Functional classification, the 
grouping of highways by the character of service they provide, was developed for transportation 
planning purposes. Comprehensive transportation planning, an integral part of total economic and 
social development, uses functional classification as an important planning tool. The emergence 
of functional classification as the predominant method of grouping highways is consistent with 
the policies contained in this publication. 



THE CONCEPT OF FUNCTIONAL CLASSIFICATION 

This section introduces the basic concepts needed for understanding the functional 
classification of highway facilities and systems. 



Hierarchies of Movements and Components 

A complete functional design system provides a series of distinct travel movements. The six 
recognizable stages in most trips include main movement, transition, distribution, collection, 
access, and termination. For example. Exhibit 1-1 shows a hypothetical highway trip using a 
freeway, where the main movement of vehicles is uninterrupted, high-speed flow. When 
approaching destinations from the freeway, vehicles reduce speed on freeway ramps, which act as 
transition roadways. The vehicles then enter moderate-speed arterials (distributor facilities) that 
bring them nearer to the vicinity of their destination neighborhoods. They next enter collector 
roads that penetrate neighborhoods. The vehicles finally enter local access roads that provide 
direct approaches to individual residences or other terminations. At their destinations the vehicles 
are parked at an appropriate terminal facility. 

Each of the six stages of a typical trip is handled by a separate facility designed specifically 
for its function. Because the movement hierarchy is based on the total amount of traffic volume, 
freeway travel is generally highest in the movement hierarchy, followed by distributor arterial 
travel, which is in turn higher in the movement hierarchy than travel on collectors and local 
access routes. 

1 



AASHTO— Geometric Design of Highways and Streets 



Main Movement 




Collection 



Exhibit 14« Hierarchy of MovemeEt 



Although many trips can be subdivided into all of the six recognizable stages, intermediate 
facilities are not always needed. The complete hierarchy of circulation facilities relates especially 
to conditions of low-density suburban development, where traffic flows are cumulative on 
successive elements of the system. However, it sometimes is desirable to reduce the number of 
components in the chain. For instance, a large single traffic generator may fill one or more lanes 
of a freeway during certain periods. In this situation, it is expedient to lead traffic directly onto a 
freeway ramp without introducing arterial facilities that unnecessarily mix already-concentrated 
traffic flows with additional vehicles. This deletion of intermediate facilities does not eliminate 
the functional need for the remaining parts of the flow hierarchy or the functional design 
components, although it may change their physical characters. The order of movement is still 
identifiable. 



The failure to recognize and accommodate by suitable design each of the different trip stages 
of the movement hierarchy is a prominent cause of highway obsolescence. Conflicts and 
congestion occur at interfaces between public highways and private traffic -generating facilities 
when the functional transitions are inadequate. Examples are commercial driveways that lead 
directly from a relatively high-speed arterial into a parking aisle without intermediate provisions 
for transition deceleration and arterial distribution or, more seriously, freeway ramps that lead 
direcfly into or from large traffic generators such as major shopping centers. 



Highway Functions 



Inadequate acceptance capacity of the distributor arterial or internal circulation deficiencies 
within the traffic absorber may lead to traffic backing up onto the freeway. Successful internal 
design that provides facilities to accommodate all the intermediate functions between the high- 
speed freeway and the terminal parking facility will alleviate such a situation. 

In the case of the freeway leading to a large traffic generator, deceleration from rapid 
movement on the freeway occurs on the exit ramp. Distribution to various parking areas is then 
accomplished by primary distribution-type roads or lanes within the parking facility. These roads 
or lanes supplant the distributor arterial function. Collector-type roads or lanes within the parking 
facility may then deliver segments of the entering flow to the parking bays. The parking aisle, in 
leading to individual parking space terminals, then becomes the equivalent of an access street. 
Thus, the principal functions within the hierarchical movement system are recognizable. In 
addition, each functional category also is related to a range of vehicle speeds. 

The same principles of design are also relevant to terminal facilities that adjoin distributor 
arterials or collectors. The functional design of the facility includes each movement stage, with 
internal circulation in the terminal design to accommodate the order of movement. The need to 
design for all stages of the movement hierarchy varies with the size of the traffic generator. For 
relatively small generators, two or more stages may be accommodated on the same internal 
facility. For larger traffic generators, each movement stage should have a separate functional 
facility. 

To determine the number of design components needed, the customary volumes of traffic 
handled by pubhc streets of different functional categories can be compared. The volume range 
on private internal facilities can be related to the comparable range on public streets. These 
volumes may not be directly comparable, inasmuch as the physical space available within a 
private facility is smaller and the operational criteria are necessarily quite different. However, the 
same principles of flow specialization and movement hierarchy can be applied. 

Some further examples may demonstrate how the principles of movement hierarchy are 
related to a logical system of classification of traffic generation intensity. At the highest practical 
level of traffic generation, a single generator fills an entire freeway, and for this condition, 
intermediate public streets could not be inserted between the generator and the freeway, so the 
various movement stages should be accommodated internally with appropriate design features. At 
the next level of traffic generation a single traffic generator could fill a single freeway lane. It is 
then appropriate to construct a freeway ramp for the exclusive use of the generator without 
intervening public streets. At still smaller volumes it becomes desirable to combine the traffic 
from several generators with additional traffic before the flow arrives at a freeway entrance ramp. 
The road performing this function then becomes a collector facility, accumulating these small 
flows until a traffic volume that will fill the freeway ramp is reached. 

Similar principles can be applied at the distributor arterial level of service. If a given traffic 
generator is of sufficient size, an exclusive intersection driveway for that generator is justified. In 
other cases an intermediate collector street should combine smaller traffic flows until they reach a 
volume that warrants an intersection along the distributor. The same theory can be applied with 
regard to the criteria for direct access to the collector street, A moderately sized traffic generator 



AASHTO — Geometric Design of Highways and Streets 



usually warrants a direct connection to the collector without an intermediate access street; 
however, in a district of single-family residences a local access street should assemble the traffic 
from a group of residences and lead it into a collector street at a single point of access, hi 
practice, direct access to arterials and collectors should be provided from commercial and 
residential properties, particularly in established neighborhoods. 

In short, each element of the functional hierarchy can serve as a collecting facihty for the 
next higher element, but an element should be present only where the intermediate collection is 
needed to satisfy the spacing needs and traffic volume demands of the next higher facility. By 
defining the spacing needs and traffic volume demands for a system element, it is possible to 
determine which cases should use the full system and in which cases intermediate elements may 
be bypassed. 



Functional Relationships 

Functional classification thus groups streets and highways according to the character of 
service they are intended to provide. This classification recognizes that individual roads and 
streets do not serve travel independently. Rather, most travel involves movement through 
networks of roads and can be categorized relative to such networks in a logical and efficient 
manner. Thus, functional classification of roads and streets is also consistent with categorization 
of travel. 

A schematic illustration of this basic idea is shown in Exhibit 1-2. In Exhibit 1-2A, lines of 
travel desire are straight lines connecting trip origins and destinations (circles). The relative 
widths of the lines indicate the relative amounts of travel desire. The relative sizes of the circles 
indicate the relative trip generating and attracting power of the places shown. Because it is 
impractical to provide direct-line connections for every desire line, trips should be channelized on 

Individual 
Forms 




City 
Town 
(A) Desire Lines of Travel 

' Local Roads 




(B) Rood Network Provided 

Exhibit 1-2, Chaeeelizatioii of Trips 



Highway Functions 



a limited road network in the manner shown in Exhibit 1-2B. Heavy travel movements are 
direcdy served or nearly so the smaller movements are channeled into somewhat indirect paths. 
The facilities in Exhibit 1-2 are labeled local access, collector, and arterial, which are terms that 
describe their functional relationships. In this scheme the functional hierarchy is also seen to be 
related to the hierarchy of trip distances served by the network. 

A more complete illustration of a functionally classified rural network is shown in 
Exhibit 1-3. The arterial highways generally provide direct service between cities and larger 
towns, which generate and attract a large proportion of the relatively longer trips. Roads of the 
intermediate functional category (collectors) serve small towns directly, connecting them to the 
arterial network. Roads of this category collect traffic from the local roads, which serve 
individual farms and other rural land uses or distribute traffic to these local roads from the 
arterials. 



v- rT. i r?Trn~i"TTT7TTrvrn 




-9L._U 






Q- 



LEGEND 
(j O Cities and Towns 

o Village 

^^^^m Arterials 

— Colladors 

— — — Locals 

Exhibit 1-3. Schematic Illustration of a Fuiictioiially Classified Rural 

Highway Network 



Although this example has a rural setting, the same basic concepts also apply in urban and 
suburban areas. A similar hierarchy of systems can be defined; however, because of the high 
intensity of land use and travel, specific travel generation centers are more difficult to identify. In 
urban and suburban areas additional considerations, such as the spacing of intersections, become 
more important in defining a logical and efficient network. A schematic illustration of a 
functionally classified suburban street network is shown in Exhibit 1-4. 



AASHTO — Geometric Design of Highways and Streets 




LEGEND 



^= Arterial Street 
--:^5^?s.Commercial Area 
= Local Street 



cio: Col lector Street 
iKSSE^ Public Area 



Exhibit 1-4, Schematic Illustration of a Portion of a Siiburbaii Street Network 



Access Needs and Controls 



The two major considerations in classifying highway and street networks functionally are 
access and mobility. The conflict between serving through movement and providing access to a 
dispersed pattern of trip origins and destinations necessitates the differences and gradations in the 
various functional types. Regulated limitation of access is needed on arterials to enhance their 
primary function of mobility. 

Conversely, the primary function of local roads and streets is to provide access 
(implementation of which causes a limitation of mobility). The extent and degree of access 
control is thus a significant factor in defining the functional category of a street or highway. 

Allied to the idea of traffic categorization is the dual role that the highway and street 
network plays in providing (1) access to property and (2) travel mobility. Access is a fixed need 
for every area served by the highway system. Mobility is provided at varying levels of service. 
Mobility can incorporate several qualitative elements, such as riding comfort and absence of 
speed changes, but the most basic factor is operating speed or trip travel time. 

Exhibit 1-2 shows that the concept of traffic categorization leads logically not only to a 
functional hierarchy of road classes but also to a similar hierarchy of relative travel distances 
served by these road classes. The hierarchy of travel distances can be related logically to 
functional specialization in meeting the property access and travel mobility needs. Local rural 
facilities emphasize the land access function. Arterials for main movement or distribution 



Highway Functions 



emphasize the high level of mobility for through movement. Collectors offer approximately 
balanced service for both functions. This scheme is illustrated conceptually in Exhibit 1-5. 

Further discussion of the various degrees of access control appropriate to street and highway 
development is provided in the section on "Access Control and Access Management" in 
Chapter 2. 

PROPORTION OF SERVICE 



l^ Mobiiity 


^ 


^^■^n^^ces^H^ 



Arterioi$ 



Cotlaotors 



Locals 



Exhibit 1-5, Relationship of Fenctionally Classified Systems in Serviiig 
Traffic Mobility and Land Access 



FUNCTIONAL SYSTEM CHARACTERISTICS 

This section contains definitions and characteristics of highway facilhies in urban and rural 
settings based on their functional classifications. It presents information, in revised form, from the 
Federal Highway Administration publication Highway Functional Classification: Concepts, 
Criteria, and Procedures (1). 



Definitions of Urban and Rural Areas 



Urban and rural areas have fundamentally different characteristics with regard to density and 
types of land use, density of street and highway networks, nature of travel patterns, and the way 
in which these elements are related. Consequently, urban and rural functional systems are 
classified separately. 



AASHTO — Geometric Design of Highways and Streets 



Urban areas are those places within boundaries set by the responsible State and local 
officials having a population of 5,000 or more. Urban areas are further subdivided into urbanized 
areas (population of 50,000 and over) and small urban areas (population between 5,000 and 
50,000). For design purposes, the population forecast for the design year should be used. (For 
legal definition of urban areas, see Section 101 of Title 23, U.S. Code.) 



Rural areas are those areas outside the boundaries of urban areas. 



Functional Categories 

The roads making up the functional systems differ for urban and rural areas. The hierarchy 
of the functional systems consists of principal arterial s (for main movement), minor arterials 
(distributors), collectors, and local roads and streets; however, in urban areas there are relatively 
more arterials with further functional subdivisions of the arterial category whereas in rural areas 
there are relatively more collectors with further functional subdivisions of the collector category. 



Functional Systems for Rural Areas 

Rural roads consist of facilities outside of urban areas. The names provided for the 
recognizable systems are principal arterials (roads), minor arterials (roads), major and minor 
collectors (roads), and local roads. 



Rural Principal Arterial System 

The rural principal arterial system consists of a network of routes with the following service 
characteristics: 

1. Corridor movement with trip length and density suitable for substantial statewide or 
interstate travel. 

2. Movements between all, or virtually all, urban areas with populations over 50,000 and a 
large majority of those with populations over 25,000. 

3. Integrated movement without stub connections except where unusual geographic or 
traffic flow conditions dictate otherwise (e.g., international boundary connections or 
connections to coastal cities). 

In the more densely populated states, this class of highway includes most (but not all) 
heavily traveled routes that might warrant multilane improvements in the majority of states; the 
principal arterial system includes most (if not all) existing rural freeways. 

The principal arterial system is stratified into the following two design types: (1) freeways 
and (2) other principal arterials. 



Highway Functions 



Rural Minor Arterial System 

The rural minor arterial road system, in conjunction with the rural principal arterial system, 
forms a network with the following service characteristics: 

1. Linkage of cities, larger towns, and other traffic generators (such as major resort areas) 
that are capable of attracting travel over similarly long distances. 

2. Integrated interstate and intercounty service. 

3. Internal spacing consistent with population density, so that all developed areas of the 
state are within reasonable distances of arterial highways. 

4. Corridor movements consistent with items (1) through (3) with trip lengths and travel 
densities greater than those predominantly served by rural collector or local systems. 

Minor arterials therefore constitute routes, the design of which should be expected to provide 
for relatively high travel speeds and minimum interference to through movement. 



Rural Collector System 

The rural collector routes generally serve travel of primarily intracounty rather than 
statewide importance and constitute those routes on which (regardless of traffic volume) 
predominant travel distances are shorter than on arterial routes. Consequently, more moderate 
speeds may be typical. To define rural collectors more clearly, this system is subclassified 
according to the following criteria: 

9 Major Collector Roads. These routes (1) serve county seats not on arterial routes, larger 
towns not directly served by the higher systems, and other traffic generators of 
equivalent intracounty importance, such as consolidated schools, shipping points, 
county parks, and important mining and agricultural areas; (2) link these places with 
nearby larger towns or cities, or with routes of higher classifications; and (3) serve the 
more important intracounty travel corridors. 

® Minor Collector Roads. These routes should (1) be spaced at intervals consistent with 
population density to accumulate traffic from local roads and bring all developed areas 
within reasonable distances of collector roads; (2) provide service to the remaining 
smaller communities; and (3) link the locally important traffic generators with their 
rural hinterland. 



Rural Local Road System 

The rural local road system, in comparison to collectors and arterial systems, primarily 
provides access to land adjacent to the collector network and serves travel over relatively short 
distances. The local road system constitutes all rural roads not classified as principal arterials, 
minor arterials, or collector roads. 



AASHTO — Geometric Design of Highways and Streets 



Extent of Rural Systems 

The functional criteria for road systems have been expressed herein primarily in qualitative 
rather than quantitative terms. Because of varying geographic conditions (e.g., population 
densities, spacing between and sizes of cities, and densities and patterns of road networks), 
criteria on sizes of population centers, trip lengths, traffic volumes, and route spacings do not 
apply to all systems in all States. However, the results of classification studies conducted in many 
States show considerable consistency (when expressed in percentages of the total length of rural 
roads) in the relative extents of the functional systems. 

Highway systems developed by using these criteria are generally expected, in all States 
except Alaska and Hawaii, to fall within the percentage ranges shown in Exhibit 1-6. The higher 
values of the ranges given in Exhibit 1-6 apply to States having less extensive total road networks 
relative to the population density. In States having more extensive total road networks relative to 
the population density, the lower values are applicable. The range of percentages of rural 
collectors represents the total length of both major and minor collector roads and applies to the 
statewide rural roadway totals the percentages in particular counties may vary considerably from 
the statewide average. Areas having an extensive regular grid pattern of roads usually have a 
smaller percentage of collectors than areas within which geographic conditions have imposed a 
restricted or less regular pattern of road development. 



Systams 


Peroeritage ©f Total Rural Road Length 


Principal arterial system 


2-4% 


Principal arterial plus minor arterial system 


6-12%, with most States falling in 7-10% range 


Collector road 


20-25% 


Local road system 


65-75% 



Exhibit 1-6. Typical Distribution of Reral Fenctioeal Systems 



Functional Highway Systems in Urbanized Areas 

The four functional highway systems for urbanized areas are urban principal arterials 
(streets), minor arterials (streets), collectors (streets), and local streets. The differences in the 
nature and intensity of development in rural and urban areas warrant corresponding differences in 
urban system characteristics relative to the correspondingly named rural systems. 



Urban Principal Arterial System 

In every urban environment, one system of streets and highways can be identified as 
unusually significant in terms of the nature and composition of travel it serves. In small urban 
areas (population under 50,000), these facilities may be very limited in number and extent, and 
their importance may be derived primarily from the service provided to through travel. In 
urbanized areas, their importance also derives from service to rurally oriented traffic, but equally 

10 



Highway Functions 



or even more importantly, from service for major circulation movements within these urbanized 
areas. 

The urban principal arterial system serves the major centers of activity of urbanized areas, 
the highest traffic volume corridors, and the longest trip desires and carries a high proportion of 
the total urban area travel even though it constitutes a relatively small percentage of the total 
roadway network. The system should be integrated both internally and between major rural 
connections. 

The principal arterial system carries most of the trips entering and leaving the urban area, as 
well as most of the through movements bypassing the central city. In addition, significant intra- 
area travel, such as between central business districts and outlying residential areas, between 
major inner-city communities, and between major suburban centers, is served by this class of 
facility. Frequently, the principal arterial system carries important intra-urban as well as intercity 
bus routes. Finally, in urbanized areas, this system provides continuity for all rural arterials that 
intercept the urban boundary. 

Because of the nature of the travel served by the principal arterial system, almost all fully 
and partially controlled access facilities are usually part of this functional class. However, this 
system is not restricted to controlled-access routes. To preserve the identification of controlled- 
access facilities, the principal arterial system should be stratified as follows: (1) interstate, 
(2) other freeways, and (3) other principal arterials (with partial or no control of access). 

The spacing of urban principal arterials is closely related to the trip-end density 
characteristics of particular portions of the urban areas. Although no firm spacing rule applies in 
all or even in most circumstances, the spacing between principal arterials (in larger urban areas) 
may vary from less than 1.6 km [1 mi] in the highly developed central business areas to 8 km [5 
mi] or more in the sparsely developed urban fringes. 

For principal arterials, service to abutting land is subordinate to travel service to major traffic 
movements. Only facilities within the subclass of other principal arterials are capable of 
providing any direct access to land, and such service should be purely incidental to the primary 
functional responsibiUty of this class of roads. 



Urban Minor Arterial Street System 

The minor arterial street system interconnects with and augments the urban principal arterial 
system. It accommodates trips of moderate length at a somewhat lower level of travel mobility 
than principal arterials do. This system distributes travel to geographic areas smaller than those 
identified with the higher system. 

The minor arterial street system includes all arterials not classified as principal. This system 
places more emphasis on land access than the higher system does and offers lower traffic 
mobility. Such a facility may carry local bus routes and provide intracommunity continuity but 
ideally does not penetrate identifiable neighborhoods. This system includes urban connections to 

11 



AASHTO— Geometric Design of Highways and Streets 



rural collector roads where such connections have not been classified as urban principal arterials 
for internal reasons. 

The spacing of minor arterial streets may vary from 0.2 to 1.0 km [0.1 to 0.5 mi] in the 
central business district to 3 to 5 km [2 to 3 mi] in the suburban fringes but is normally not more 
than 2 km [1 mi] in fully developed areas. 



Urban Collector Street System 

The collector street system provides both land access service and traffic circulation within 
residential neighborhoods and commercial and industrial areas. It differs from the arterial system 
in that facilities on the collector system may penetrate residential neighborhoods, distributing 
trips from the arterials through the area to their ultimate destinations. Conversely, the collector 
street also collects traffic from local streets in residential neighborhoods and channels it into the 
arterial system. In the central business district, and in other areas of similar development and 
traffic density, the collector system may include the entire street grid. The collector street system 
may also carry local bus routes. 



Urban Local Street System 

The local street system comprises all facilities not in one of the higher systems. It primarily 
permits direct access to abutting lands and connections to the higher order systems. It offers the 
lowest level of mobility and usually contains no bus routes. Service to through-traffic movement 
usually is deliberately discouraged. 



Length ©f Roadway and Travel on Urban Systems 

Exhibit 1-7 contains the typical distribution of travel volume and length of roadway of the 
functional systems for urbanized areas. Systems developed for urbanized areas using the criteria 
herein usually fall within the percentage ranges shown. 



Systems 


Range 


Travel volume (%) 


Length (%) 


Principal arterial system 

Principal arterial plus minor arterial street system 

Collector road 

Local road system 


40-65 

65-80 

5-10 

10-30 


5-10 

15-25 

5-10 

65-80 



Exhibit l-7» Typical Distribution of Urban Functional Systems 



72 



Highway Functions 



Functional Classification as a Design Type 

This text has utihzed the functional classification system as a design type of highway. Two 
major difficulties arise from this usage. The first major problem involves freeways. A freeway is 
not a functional class in itself but is normally classified as a principal arterial. It does, however, 
have unique geometric criteria that demand a separate design designation apart from other 
arterials. Therefore, a separate chapter on freeways has been included along with chapters on 
arterials, collectors, and local roads and streets. The addition of the universally familiar term 
"freeway" to the basic functional classes seems preferable to the adoption of a complete separate 
system of design types. 

The second major difficulty is that, in the past, geometric design criteria and capacity levels 
have traditionally been based on a classification of traffic volume ranges. Under such a system, 
highways with comparable traffic volumes are constructed to the same criteria and provide 
identical levels of service, although there may be considerable difference in the functions they 
serve. 

Under a functional classification system, design criteria and level of service vary according 
to the function of the highway facility. Volumes serve to further refine the design criteria for each 
class. 

Arterials are expected to provide a high degree of mobility for the longer trip length. 
Therefore, they should provide a high operating speed and level of service. Since access to 
abutting property is not their major function, some degree of access control is desirable to 
enhance mobility. The collectors serve a dual function in accommodating the shorter trip and 
feeding the arterials. They should provide some degree of mobility and also serve abutting 
property. Thus, an intermediate design speed and level of service is appropriate. Local roads and 
streets have relatively short trip lengths, and, because property access is their main function, there 
is little need for mobility or high operating speeds. This function is reflected by use of a lower 
design speed and level of service. 

The functional concept is important to the designer. Even though many of the geometric 
design values could be determined without reference to the functional classification, the designer 
should keep in mind the overall purpose that the street or highway is intended to serve. This 
concept is consistent with a systematic approach to highway planning and design. 

The first step in the design process is to define the function that the facility is to serve. The 
level of service needed to fulfill this function for the anticipated volume and composition of 
traffic provides a rational and cost-effective basis for the selection of design speed and geometric 
criteria within the ranges of values available to the designer. The use of functional classification 
as a design type should appropriately integrate the highway planning and design process. 



13 



AASHTO — Geometric Design of Highways and Streets 



REFERENCES 

1. U.S. Department of Transportation, Federal Highway Administration. Highway 
Functional Classification: Concepts, Criteria, and Procedures, Washington, D.C.: 
1989. 

2. U.S. Department of Transportation, Federal Highway Administration, Office of 
Information Management. Our Nation's Highways — Selected Facts and Figures, Report 
No, FHWA-FL-98-015, Washington, D.C.: 1998. 



14 



CHAPTER 2 
DESIGN CONTROLS AND CRITERIA 

INTRODUCTION 

This chapter discusses those characteristics of vehicles, pedestrians, and traffic that act as 
criteria for the optimization or improvement in design of the various highway and street 
functional classes. 



DESIGN VEHICLES 
General Characteristics 

Key controls in geometric highway design are the physical characteristics and the 
proportions of vehicles of various sizes using the highway. Therefore, it is appropriate to 
examine all vehicle types, establish general class groupings, and select vehicles of representative 
size within each class for design use. These selected vehicles, with representative weight, 
dimensions, and operating characteristics, used to establish highway design controls for 
accommodating vehicles of designated classes, are known as design vehicles. For purposes of 
geometric design, each design vehicle has larger physical dimensions and a larger minimum 
turning radius than most vehicles in its class. The largest design vehicles are usually 
accommodated in freeway design. 

Four general classes of design vehicles have been established: (1) passenger cars, (2) buses, 
(3) trucks, and (4) recreational vehicles. The passenger-car class includes passenger cars of all 
sizes, sport/utility vehicles, minivans, vans, and pick-up trucks. Buses include inter-city (motor 
coaches), city transit, school, and articulated buses. The truck class includes single-unit trucks, 
truck tractor-semitrailer combinations, and truck tractors with semitrailers in combination with 
full trailers. Recreational vehicles include motor homes, cars with camper trailers, cars with boat 
trailers, motor homes with boat trailers, and motor homes pulling cars. In addition, the bicycle 
should also be considered as a design vehicle where bicycle use is allowed on a highway. 

Dimensions for 19 design vehicles representing vehicles within these general classes are 
given in Exhibit 2-1. In the design of any highway facility, the designer should consider the 
largest design vehicle likely to use that facility with considerable frequency or a design vehicle 
with special characteristics appropriate to a particular intersection in determining the design of 
such critical features as radii at intersections and radii of turning roadways. In addition, as a 
general guide, the following may be considered when selecting a design vehicle: 

® A passenger car may be selected when the main traffic generator is a parking lot or 

series of parking lots. 
^ A single-unit truck may be used for intersection design of residential streets and park 

roads. 



15 



AASHTO^Geometric Design of Highways and Streets 







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Ii II II II ii II II II 



16 



Design Controls and Criteria 






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I) It )t II II 11 II II It 



17 



AASHTO — Geometric Design of Highways and Streets 



® A city transit bus may be used in the design of state highway intersections with city 
streets that are designated bus routes and that have relatively few large trucks using 
them. 

® Depending on expected usage, a large school bus (84 passengers) or a conventional 
school bus (65 passengers) may be used for the design of intersections of highways with 
low-volume county highways and township/local roads under 400 ADT. The school bus 
may also be appropriate for the design of some subdivision street intersections. 

® The WB-20 [WB-65 or 67] truck should generally be the minimum size design vehicle 
considered for intersections of freeway ramp terminals with arterial crossroads and for 
other intersections on state highways and industrialized streets that carry high volumes 
of traffic and/or that provide local access for large trucks. 

In addition to the 19 design vehicles, dimensions for a typical farm tractor are shown in 
Exhibit 2-1, and the minimum turning radius for a farm tractor with one wagon is shown in 
Exhibit 2-2. Turning paths of design vehicles can be determined from the dimensions shown in 
Exhibit 2-1 and 2-2 and through the use of commercially available computer programs. 



iVIinimum Turning Paths of Design Vehicles 

Exhibits 2-3 through 2-23 present the minimum turning paths for 19 typical design vehicles. 
The principal dimensions affecting design are the minimum centerline turning radius (CTR), the 
out-to-out track width, the wheelbase, and the path of the inner rear tire. Effects of driver 
characteristics (such as the speed at which the driver makes a turn) and of the slip angles of 
wheels are minimized by assuming that the speed of the vehicle for the minimum turning radius is 
less than 15 km/h [10 mph]. 

The boundaries of the turning paths of each design vehicle for its sharpest turns are 
established by the outer trace of the front overhang and the path of the inner rear wheel. This tum 
assumes that the outer front wheel follows the circular arc defining the minimum centerline 
turning radius as determined by the vehicle steering mechanism. The minimum radii of the 
outside and inside wheel paths and the centerline turning radii (CTR) for specific design vehicles 
are given in Exhibit 2-2. 

Trucks and buses generally require more generous geometric designs than do passenger 
vehicles. This is largely because trucks and buses are wider and have longer wheelbases and 
greater minimum turning radii, which are the principal vehicle dimensions affecting horizontal 
alignment and cross section. Single-unit trucks and buses have smaller minimum turning radii 
than most combination vehicles, but because of their greater offtracking, the longer combination 
vehicles need greater turning path widths. Exhibit 2-11 defines the turning characteristics of a 
typical tractor/semitrailer combination. Exhibit 2-12 defines the lengths of tractors commonly 
used in tractor/semitrailer combinations. 

A combination truck is a single-unit truck with a full trailer, a truck tractor with a 
semitrailer, or a truck tractor with a semitrailer and one or more full trailers. Because combination 
truck sizes and tuming characteristics vary widely, there are several combination truck design 

18 



Design Controls and Criteria 



Metric 



Design 

Vehlcre 

Type 


Pas- 

senger 
Car 


Single 

Unit 

Ttuck 


Inter-clty Bus 
(Motor Coacti) 


City 
Transit 

Bus 


Conven- 
tional 
School 
Bus (65 


Large^ 
School 
Bus (84 


Articu- 
lated Bus 


Intermed- 
iate Seml- 
tratter 


intermed^ 
late Semi- 
trailer 


Symbol 


P 


su 


BUS-12 


BUS-14 


CITY-BUS 


S-BUS11 


S-BUS12 


A-BUS 


WB-12 


WB-15 


Minimum 
Design 
Turning 

Radius 
(m) 


7.3 


12.8 


13.7 


13.7 


12.8 


11.9 


12.0 


12.1 


12.2 


13.7 


Center- 
line' 
Turning 
Radius 
(CTR) (m) 


6.4 


11.6 


12.4 


12.4 


11.5 


10.6 


10.8 


10.8 


11.0 


12.5 


Minimum 

Inside 

Radius 

(m) 


4.4 


8.6 


8,4 


7.8 


7.5 


7.3 


7.7 


6.5 


5.9 


5.2 


Design 

Vehicle 

Typo 


Inter 
Semi- 


State 

traHer i 


"Doufel© 
Bottom'' 
Combina- 
tion 


Triple 
Semi- 
trailer/ 

traNers 1 


Turnpike 
Double 
Semi- 
trailer/ 
trailer 


; i.. ....... ...... 


Car and 

.M<aii«?< 


Car and 

Boat 

Trailer 


Motor 

Homo 

and Boat 

Trailer 


Farm 
Tractor 
w/One 
Wagon 


Symbol 


WB-19* 


WB-20** 


WB-20D 


WB-30T 


WB-33D* 


MH 


PfT 


P/B 


MH/B 


TR/W 


Minimum 

Design 

Turning 

Radius 

(m) 


13.7 


13.7 


13.7 


13.7 


18.3 


12.2 


10.1 


7.3 


15.2 


5.5 


Center- 
line^ 

Turning 

Radius 

(CTR) (m) 


12.5 


12.5 


12.5 


12.5 


17.1 


11.0 


9.1 


6.4 


14.0 


4.3 


Minimum 

Inside 

Radius 

(m) 


2.4 


1.3 


5.9 


3.0 


4.5 


7.9 


5.3 


2.8 


10.7 


3.2 



Note: Numbers In table have been rounded to the nearest tenth of a meter. 

= Design vehicle with 1 4.63 m trailer as adopted in 1 982 Surface Transportation Assistance Act (STAA). 
= Design vehicle with 1 6.1 6 m trailer as grandfathered in with 1 982 Surface Transportation Assistance Act (STAA). 
^ = The turning radius assumed by a designer when investigating possible turning paths and is set at the centerline of 

the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum 

design turning radius minus one-half the front width of the vehicle. 
^ = School buses are manufactured from 42 passenger to 84 passenger sizes. This corresponds to wheelbase 

lengths of 3,350 mm to 6,020 mm, respectively. For these different sizes, the minimum design turning radii vary 

from 8.78 m to 12.01 m and the minimum inside radii vary from 4.27 m to 7.74 m. 
^ = Turning radius is for 150-200 hp tractor with one 5.64 m long wagon attached to hitch point. Front wheel drive is 

disengaged and without brakes being applied. 



Exhibit 2-2c Miiumtim Terniag Radii of Design Vehicles 



19 



AASHTO — Geometric Design of Highways and Streets 



US Customary 



Design 

Vehfolo 

Type 


Pas- 
senger 
Car 


Single 

Unit 
Truck 


Inter-City Bus 
(Motot Coach) 


City 

Transit 

Bus 


Conven- 
tional 
School 
Bus (65 


Large' 
School 
Bus(B4 
pass.) 


ArtJcu- 
fated Bus^ 


Intermed- 
iate Semi- 
trailer 


Intermed* 
tate Semi- 
trailer 


Symooi 


P 


SU 


BUS-40 


BUS-45 


CITY-BUS 


S-BUS36 


S-BUS40 


A-BUS 


WB-40 


WB-50 


Minimum 

Design 

Tuming 

Radius 

(ft) 


24 


42 


45 


45 


42.0 


38.9 


39.4 


39.8 


40 


45 


Center- 
line' 
Tuming 
Radius 
(CTR) (ft) 


21 


38 


40.8 


40.8 


37.8 


34.9 


35.4 


35.5 


36 


41 


Minimum 

Inside 

Radius 

(ft) 


14.4 


28.3 


27.6 


25.5 


24.5 


23.8 


25.4 


21.3 


19.3 


17.0 


Design 

Vehicle 

Type 


Interstate SemM 

weHBT 


"Double 
Bottom" 
Combina- 
tion 


Triple 
Sefni° 
tiBller/ 
trailers 


Turnpike 

Double 

Semi- 

trailef/ 

trailer 


iiiiii 


iisir and 
iigimper 

i^-iralter:,:,:! 


Car and 
Boat 

..■Trailer. 


1 Motor 

lid Boat 
Trailer 


Farmf 
iTractor 

w/One 
1 Wagon 


Symbol 


WB-62* 


WB-65** 
orWB-67 


WB-67D 


WB-100T 


WB-109D* 


MH 


P/T 


P/B 


MH/B 


TRW 


Minimum 

Design 

Turning 

Radius 

(ft) 


45 


45 


45 


45 


60 


40 


33 


24 


50 


18 


Center- 
tine' 
Turning 
Radius 
(CTR) (ft) 


41 


41 


41 


41 


56 


36 


30 


21 


46 


14 


Minimum 

Inside 

Radius 

(ft) 


7.9 


4.4 


19.3 


9.9 


14.9 


25.9 


17.4 


8.0 


35.1 


10.5 



Design vehicle with 48 ft trailer as adopted in 1982 Surface Transportation Assistance Act (STAA). 

Design vehicle with 53 ft trailer as grandfathered in with 1 982 Surface Transportation Assistance Act (STAA). 

The turning radius assumed by a designer when investigating possible turning paths and is set at the centerline of 

the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum 

design turning radius minus one-half the front width of the vehicle. 

School buses are manufactured from 42 passenger to 84 passenger sizes. This corresponds to wheelbase 

lengths of 132 in to 237 in, respectively. For these different sizes, the minimum design turning radii vary from 

28.8 ft to 39.4 ft and the minimum inside radii vary from 14.0 ft to 25.4 ft. 

Turning radius is for 1 50-200 hp tractor with one 1 8.5 ft long wagon attached to hitch point. Front wheel drive is 

disengaged and without brakes being applied. 

Exhibit 2-2» Mtnimiiiii Turning Radii of Design Vehicles (Continued) 



20 



Design Controls and Criteria 




^ 152 m 




3.35 m 


0.91 m 


[5ftl 


5.79 m 


[11 tt] 


[3ft] 

^ 




[19 ft] 







5ft 



10ft 



T 



1m 



2.5 m 



scale 



Pat h of left 
front wheel 



Patfi of front 




Exhibit 2-3. Minimum Turning Path for Passenger Car (P) Design Vehicle 



21 



AASHTO—Geometric Design of Highways and Streets 



m — . 







5ft 10ft 




1m 2.5 m 
scale 



[30 ft] 



Path of front 



P ath of left 
front wheel 




2.44 m 
[8 ft] 



• Assumed steering angle is 31 .7 

• CTR = Centeriine turning 
radius at front axle 



Exhibit 2-4. Minimum Turning Path for Single-Unit (SU) Truck Design Vehicle 



22 



Design Controls and Criteria 




1.92m ,1.13,nri 



7.32 m 



(6.3 ft] [3.7 ftj 12.20 m 1^4 ft] 



[40 ft] 



--^»|««^ 



1.83 m 



[6 ft] 



5ft 10 ft 



1m 2.5 m 

scale 



P ath of left 
front wheel 



[8.5 ft] 



^ Assumed steering angle is 38:7* 
# CTR = Centerline turning 
radius at front axle 



Path of front 




Exhibit 2-5. Minimem Turning Path for Intercity Bus (BUS-12 [BUS-40]) Design Vehicle 



23 



AASHTO — Geometric Design of Highways and Streets 




145 ft] 



Q 5ft 10 ft 

1 m 2,5 m 
scale 



Path of front 



Path of (aft 
front wheel 




2.59 m* 
[8-5 ft] 



2.5 m 
scale 



^ Assumed steering angle is 44.4 
# CTR = Centertine turning 

radius at front axle 



Exhibit 2-6. Miiiimum Tiirnieg Path for Intercity Bus (BUS-14 [BUS-45]) Design Vehicle 



24 



Design Controls and Criteria 




|40 ft] 



5ft 10 ft 

" 1 — :"Z3 



1m 2.5 m 
scale 



PMhoflaft 
fron 



Path of fro nt 
^ ^ ovartiang 




2.59 m 
[8.5 ft] 



Assumed steering angle is 41 
GTR = Centeriine turning 
radius at front axle 



Exhibit 2-7. Minimum Turning Path for City Transit Bus (CITY-BUS) Design Vehicle 



25 



AASHTO — Geometric Design of Highways and Streets 




(Vista Style) 



10 ft 



1 m 2.5 m 



P ath of left 
front wheel 



Path of front 




® Assumed steering angle is 37.2^ 

^ CTR- = Genteriirw tyrniog 

radlys at front axle 
^ 65 passenger bus 



Exhibit 2-8, Miiiimiim Turniirig Path for Conventional School Bus (S-BUS»11 [S-BUS-36]) 

Design Vehicle 



26 



Design Controls and Criteria 



b 



t } t ' I f 1 n n n rr n n t — i 



IBb 



3,Mm 



{13 ft] 



=iOt= 



-*4-* 



6.10m 



12.20 m 120 ft] 



HO ft] 

(Transit sfyle) 



:^ 




mama 



, , i,13m 



17 ft] 



Q Sft 10 ft 
1m 2.5 m 



Palii0f!aft 

frorS 



Assumed steerini angle is 34.2 
CTR ^ Centsrlina turning 
radios at front axle 
84 passer^ger bus 



Pati of front 




Exhibit 2-9. Minimum Turaiiig Path for Large School Bus (S-BUS-12 [S-BUS-40]) 

Design Vehicle 



27 



AASHTO — Geometric Design of Highways and Streets 



&Z1 



5ft 



10ft 



1m 2.5 m 

scale 



1 ^1.22 m 




[60,0 fl] 



Path of let 
front wliiil 



^ Assumed steering angle !s 3T.8 
^^MM\ ^ Articulated angle is 38. 1 ^ 
[8.5 ft] « ^^^ - Centeriine turning 
radius at front axle 



Path of front 




Exhibit 240, Minimum Turniog Path for Articulated Bus (A-BUS) Design Vehicle 



28 



Design Controls and Criteria 



Path of Overhang 




Tractor /traiter 
angle 



f Angle 



Turning Radiys . ^ . 

— ~ — - — — ^ .^ Turning 



Path of Front Inside 



Tractor Tire 



Center 



Definitions: 

1. Turning radius — The circular arc formed by the turning path radius of the front outside tire of a vehicle. 

This radius is also described by vehicle manufacturers as the 'lurning curb radius." 

2. CTR — The turning radius of the centerllne of the front axle of a vehicle. 

3. Offtracking — The difference in the paths of the front and rear wheels of a tractor/semitrailer as it 

negotiates a turn. The path of the rear tires of a turning truck does not coincide with that of the 
front tires, and this effect is shown in the drawing above. 

4. Swept path width — The amount of roadway width that a truck covers in negotiating a turn and is equal 

to the amount of offtracking plus the width of the tractor unit. The most significant dimension 
affecting the swept path width of a tractor/semitrailer is the distance from the kingpin to the rear 
trailer axle or axles. The greater this distance is, the greater the swept path width. 

5. Steering angle— The maximum angle of turn built into the steering mechanism of the front wheels of a 

vehicle. This maximum angle controls the minimum turning radius of the vehicle. 

6. Tractor/trailer angle — The angle between adjoining units of a tractor/semitrailer when the combination 

unit is placed into a turn; this angle is measured between the longitudinal axes of the tractor and 
trailer as the vehicle turns. The maximum tractor/trailer angle occurs when a vehicle makes a 180" 
turn at the minimum turning radius; this angle is reached slightly beyond the point where maximum 
swept path width is achieved. 

Exhibit 2-11, Turning Characteristics of a Typical Tractor-Semitrailer Combination Truck 



29 



AASHTO — Geometric Design of Highways and Streets 



a. Typical long-hayl tractom (Trador - semitraifer configuration} 

^ 3-51 m _. 

■^ .,^,.,,i, .„. .... . > >..,. i ^_M , ^ 



(GABOVER) 




^ 4,57 m 



(CONVEMTIOHAL) 



[4,33 ft] 



[4.42 ft] 



[12.2 ft] 




[19,5 ft] 



b. Typical city m6 sliorl-hauf tractors (Tractor • semitrailer configufation) 

2.29m 



_ 2,29 m 




0.71 m 
[2.33 ft] 



[12,5 ft) 



[4.42 ft] 



[12.5 ft] 



c. Typical tractor for Double ^ Triple configuration 



1.98 m 




[2,33 ft] 



[11 ft] 



[233 ft) 



d. Typical tractor for Rocky youmain Double ^ Turnpike Double configuration 



1.98.m 



(CA80VER) 



_ 2,89 m ^^ 




0J1 m 

2.33 ft) 



[4.42 ft] 



[122 ft] 



[4.42 ft] 



[17.5 ft) 



Exhibit 2-12. Lengths of Commonly Used Truck Tractors 



30 



Design Controls and Criteria 



1ft(mmi33ft]Traijar 




12.20 m [40 ft] Wheetoase 



13.87 m[45>5 ftjor greater 



0,91 m 



^ Typical tire size and space between 

tires applies to all trailem. 



Path of left 



Path of f ront 
overtiang 




IHI 



® Assumed steering angle is 20,4 ^ 
« Assumed traGtor/irailer angle is 46 ^ 
® GTR " Centerlina turning 
radios at front axle 



2.44 m 
[8.0 ft] 

Exhibit 2-13. Minimum Turning Path for Intermediate Semitrailer (WB-12 [WB-40]) 

Design Vehicle 



31 



AASHTO — Geometric Design of Highways and Streets 



1.22 m 



Mk 







12.9Sfn [42.5 ft] Trailer 



10.82 m [35.5 ftl 



5ft 10 ft ! 

p^i , I J 

1m 2.5 m 0.91 m^j 

scale [3 ft] | 



T 







.1.22 m 



[4 ft] 



■128'?- 3.17 m ^ 
{4.2 ft] [10.4 ft] 
1.35 rn| 3 81m _| 



15,24 m [50 ft] Wheelbase t^-^^ft] [12.5 ft] J 
16J7m [55 ft] or greater 1 




Path of front 



Path of left 




» Assumed steering angle is 17.9*^ 
* Assumed tractor/trailer angle is 56** 
^ CTR = Centeriine turning radius 
at front axle 



[8,5 ft] 

Exhibit 2-14, Minimum Tiirning Path for Intermediate Semitrailer (WB-15 [WB-50]) 

Design Vehicle 



2.59 m 



32 



Design Controls and Criteria 



14.63 in {48 ffl Trailer 




[4 ft] 



IBM m [62 ft] Wheelbase [1 9.5 ft) J 

mmmmsm i 



[4 ft] 



P att of left 
frontwheel 



Path of front 




Exhibit 2-15, Mioimem Turning Path for Interstate Semitrailer (WB»19 [WB-62]) 

Design Vehicle 



33 



AASHTO — Geometric Design of Highways and Streets 



iaiSm[53ftlTrall©r 




14.42 ft] 
19.81 m [65 ft] Wheelbase 






5,30 m 

S 95 mnT-4 ft] 



122 m 



11 as ft) 



22.41 m [73.5 ft] 



P ath of \en 
front wh^r 



Pathoftcmt 




Exhibit 2-16. Minimiim Tiimieg Path for Interstate Semitrailer 
(WB-20 [WB»65 and WB-67]) Design Vehicle 



34 



Design Controls and Criteria 



a,S9mt2a;5t1Trailar 



aJ9m(.28.,SftlTmtl© -f 




2.44 m 






wm 



pi 



im 



5ft 1011 

ill 1 


f ft. ,1 

2Jm 

^^^^ Path of fight/' 

rear wheel 


Assym:ei siBering angle Is 15,7 ^ 
Assumed tractor/lfailer a:ngle Is 35. 1 ^ 
M^ymed traller/lraifef angia is S6.0 ^ 
CT.R « Centefliria turning 
radfus at trout axle 



2,59 m 
{8.5 ft] 



Exhibit 2=17. Minimiiiii Turning Path for DouMe-Trailer Combination 
(WB-20D [WB-67D]) Design Vehicle 



35 



AASHTO — Geometric Design of Highways and Streets 



8.69 m[28.5 ft] Trailer 



. 7.01m [ 23 ftj 



1 



I 091 m Q-76"^> ' 
fwi I2-5ft] ' 



w 



8.69 m [28.5 ft] T railef 



r 



7.01 m [23 ft] 



j 0.91 m 0"^^''". . 
[2.5 ft] 




8.69 m[28.5 ft] Trailer 



7.01m [23 ft] 



-0.91 m 



0.76 m„ I ^' 
{2.5 ft] ' 



1.98 m 



Fft] ■^6;86m[22.5ff] 
30.33 m [99.5 ft] Wheelbase 



31.96 m [104.83 ft] 




0.71 m 
[2.33ft] 



5ft 10ft 

2.5 m 
scale 



Path of front 



Path of left 




Exhibit 2-18. Minimum Turning Path for Triple-Trailer Combination 
(WB-30T [WB-IOOT]) Design Vehicle 



36 



Design Controls and Criteria 



tZJ m 12-34 m (40.5 ft] 



14.63 m{48 ft] Trailer 






h 



U UMjp 

[4 ft] 



0.91 JTi 



5 



WIT 

ftioftT wm 



2.5 m 
scale 



14.63 mr48ft1 Trailer 




12.34 mf40.Sft1 



0.91 1 



1.28 ml^l'^ 



M2ir 
1.35 m 



12.19 m [COT >.|Tl2.2l'i 
33.29 mf 109.2 ft lWhedbase [-^0 »] ^ | 
34.77 m [114.03 ftl , 




p33ft] 



Path of left 



Path of front 




2.59 m 
[8.5 ft] 

Exhibit 2-19. Minimum Turning Path for Turnpike-Double Combination 
(WB-33D [WB-109D]) Design Vehicle 



37 



AASHTO — Geometric Design of Highways and Streets 




[30 ft] 



5ft 10 ft 

1 m 2.5 m 
scale 



Path of left 
front wheel 




Path of front 
overhang 



* Assumed st^ririg angle is 33 J° 
^ CTR = Centertine turning 
radius at front axle 

Exhibit 2-20, Miiiimiim Tiirnieg Path for Motor Home (MH) Design Vehicle 



38 



Design Controls and Criteria 



[-* 


8.23 m [27 ft] 


— ^- 


/m_ 





--:i\ 




10ft 



1 m 2.6 m 

scale 



Path of 
front wheal 




Path of front 
overhang 



« 



^ Assumed sttaring angle ts 21 ;6 ^ 
« Assumed c^r/traiter angle is 47.2*^ 
^ CTR =^ Centerllne turning 

radiy s at front axle 



1 I 

2.44 m 

18 ft] 

Exhibit 2-21. Minimum Turning Path for Passenger Car and Camper Trailer (P/T) 

Design Vehicle 



39 



AASHTO — Geometric Design of Highways and Streets 



6/1 m 




2.44 m 

^ - r: - " ^^ 



18 ft] 



0.91 m. I 
4.57 m 



ft 5 ft] 



1-52 ml 

^ ;" | "P j»|ii^^ 



{5 ft] 



3.35 m 
5.79 m 



"^^l!s4 



119 ft] 



5ft 10 ft 



i: 



J 




1m 2.5 m 
scale 



Palh of front 



18 ft] 



« Assymad steering angle is 316^ 
® Assumed caT/tratter angle is 611 *^ 
* CTR = Gentarrme lurning 
radjys at front axle 



Exhibit 2-22, Minimum TerniEg Path for Passenger Car and Boat Trailer (P/B) 

Design Vehicle 



40 



Design Controls and Criteria 



6.1m 



[20 fl] 



CD D 



m 




5ft 10 ft 



Path of left 
front wheel 



1 m 2.5 m 
scale 

Path of front 
overhang 




h—H 



* Assumed steering angle is 25.8^ 

^ Assumed motor home/trailer angle is 30^ 

* CTR = Gen^rline turning 
radius at front axle 



[8ft] 

Exhibit 2-23« Minimiim Turning Path for Motor Home and Boat Trailer (MH/B) 

Design Vehicle 



u 

2.44 m 



41 



AASHTO — Geometric Design of Highways and Streets 



vehicles. These combination trucks are identified by the designation WB, together with the wheel 
base or another length dimension in both metric and U.S. customary units. The combination truck 
design vehicles are: (1) the WB-12 [WB-40] design vehicle representative of intermediate size 
tractor-semitrailer combinations, (2) the WB-15 [WB~50] design vehicle representative of a 
slightly larger intermediate size tractor-semitrailer combination commonly in use, (3) the WB-19 
[WB-62] design vehicle representative of larger tractor semitrailer combinations allowed on 
selected highways by the Surface Transportation Assistance Act of 1982, (4) the WB-20 [WB-65 
or WB-67] design vehicle representative of a larger tractor-semitrailer allowed to operate on 
selected highways by "grandfather" rights under the Surface Transportation Assistance Act of 
1982, (5) the WB-20D [WB-67D] design vehicle representative of a tractor-semitrailer/full trailer 
(doubles or twin trailer) combination commonly in use, (6) the WB-30T [WB-IOOT] design 
vehicle representative of tractor-semitrailer/full trailer/full trailer combinations (triples) 
selectively in use, and (7) the WB-33D [WB-109D] design vehicle representative of larger 
tractor-semitrailer/full trailer combinations (turnpike double) selectively in use. Although 
turnpike doubles and triple trailers are not permitted on many highways, their occurrence does 
warrant inclusion in this publication. 

The minimum turning radii and transition lengths shown in the exhibits are for turns at less 
than 15 km/h [10 mph]. Longer transition curves and larger curve radii are needed for roadways 
with higher speeds. The radii shown are considered appropriate minimum values for use in 
design, although skilled drivers might be able to turn with a slightly smaller radius. 

The dimensions of the design vehicles take into account recent trends in motor vehicle sizes 
manufactured in the United States and represent a composite of vehicles currently in operation. 
However, the design vehicle dimensions are intended to represent vehicle sizes that are critical to 
geometric design and thus are larger than nearly all vehicles belonging to their corresponding 
vehicle classes. 

The turning paths shown in Exhibits 2-3 through 2-10 and Exhibits 2-13 through 2-23 were 
derived by using commercially available computer programs. 

The P design vehicle, with the dimensions and turning characteristics shown in Exhibit 2-3, 
represents a larger passenger car. 

The SU design vehicle represents a larger single-unit truck. The control dimensions indicate 
the minimum turning path for most single-unit trucks now in operation (see Exhibit 2-4). On 
long-distance facilities serving large over-the-road truck traffic or inter-city buses (motor 
coaches), the design vehicle should generally be either a combination truck or an inter-city bus 
(see Exhibit 2-5 or Exhibit 2-6). 

For intra-city or city transit buses, a design vehicle designated as CITY-BUS is shown in 
Exhibit 2-7. This design vehicle has a wheel base of 7.62 m [25 ft] and an overall length of 
12.20 m [40 ft]. Buses serving particular urban areas may not conform to the dimensions shown 
in Exhibit 2-7. For example, articulated buses, which are now used in certain cities, are longer 
than a conventional bus, with a permanent hinge near the vehicle's center that allows more 
maneuverability. Exhibit 2-10 displays the critical dimensions for the A-BUS design vehicle. 

42 



Design Controls and Criteria 



Also, due to the importance of school buses, two design vehicles designated as S-BUS 11 
[S-BUS 36] and S-BUS 12 [S-BUS 40] are shown in Exhibits 2-8 and 2-9, respectively. The 
larger design vehicle is an 84-passenger bus and the smaller design vehicle is a 65 -passenger bus. 
The highway designer should also be aware that for certain buses the combination of ground 
clearance, overhang, and vertical curvature of the roadway may present problems in hilly areas. 

Exhibits 2-13 through 2-19 show dimensions and the minimum turning paths of the design 
vehicles that represent various combination trucks. For local roads and streets, the WB-15 
[WB-50] or WB-12 [WB-40] is often considered an appropriate design vehicle. The larger 
combination trucks are appropriate for design of facilities that serve over-the-road trucks. 

Exhibits 2-20 through 2-23 indicate minimum turning paths for typical recreational vehicles. 

In addition to the vehicles shown in Exhibits 2-3 through 2-10 and Exhibits 2-13 through 
2-23, other vehicles may be used for selected design applications, as appropriate. With the advent 
of computer programs that can derive turning path plots, the designer can determine the path 
characteristics of any selected vehicle if it differs from those shown (1). 



Vehicle Performance 

Acceleration and deceleration rates of vehicles are often critical parameters in determining 
highway design. These rates often govern the dimensions of such design features as intersections, 
freeway ramps, climbing or passing lanes, and turnout bays for buses. The following data are not 
meant to depict average performance for specific vehicle classes but rather lower performance 
vehicles suitable for design application, such as a low-powered (compact) car and a loaded truck 
or bus. 

Based on its acceleration and deceleration performance, the passenger car seldom controls 
design. From Exhibits 2-24 and 2-25, it is obvious that relatively rapid accelerations and 
decelerations are possible, although they may be uncomfortable for the vehicle's passengers. 
Also, due to the rapid changes being made in vehicle operating characteristics, current data on 
acceleration and deceleration may soon become outdated. In addition, refer to the NCHRP 
Report 400, Determination of Stopping Sight Distances (2). Exhibit 2-24 is based on NCHRP 
Report 270 (3). 

When a highway is located in a recreational area, the performance characteristics of 
recreational vehicles should be considered. 



Vehicular Pollution 

Pollutants emitted from motor vehicles and their impact on land uses adjacent to highways 
are factors affecting the highway design process. As each vehicle travels along the highway, it 
emits pollutants into the atmosphere and transmits noise to the surrounding area. The highway 



43 



AASHTO — Geometric Design of Highways and Streets 



METRIC 




m ^^ ^m m> ^^ ^^ 

PASSEHOER CARS - DiSTANCE TRAVELED - METERS 



US CUSTOMARY 




PASSENGER CARS - DISTANCE TRAVEIHD - FEET 



Exhibit 2-24, Acceleration of Passenger Cars, Level Conditions 



44 



Design Controls and Criteria 



METRIC 



1 1 






|l "' 



it 




^ i // . ' < ^^ — -«• 



SPEEO REACHED 

(COMFOHTABlE RATE) 






M«Ni*i/»UM BRAKJNG DISTANCE 



X ^ DRY PAVEMENt 
Y^WETPAVEyEMT 



US CUSTOMARY 



IS 

if 
^1 




^Q mi im im 



SPEED REACHED 

(COMFORTABLE RATE) 

A = 50 mph 
B » 40 ft^h 
C ^ 30 mph 
D - 20 mph 
E - rr^h 

MIMIMUM BRAKING DISTANCE 

X- DRY PAVEMENT 

Y== WET PAVEMENT 



Exhibit 2-25. Deceleration Distances for Passenger Vehicles Approaching Intersections 



designer should recognize these impacts and evaluate them in selecting appropriate transportation 
alternatives. Many factors affect the rate of pollutant emission from vehicles, including vehicle 
mix, vehicle speed, ambient air temperature, vehicle age distribution, and percentage of vehicles 
operating in a cold mode. 

In addition to air pollution, the highway designer should also consider noise pollution. Noise 
is unwanted sound, a subjective result of sounds that intrude on or interfere with activities such as 
conversation, thinking, reading, or sleeping. Thus, sound can exist without people — noise cannot. 

Motor vehicle noise is generated by the mechanical operation of the vehicle and its 
equipment, by its aerodynamics, by the action of its tires on the pavement, and, in metropolitan 
areas, by the short-duration sounds of brake squeal, exhaust backfires, horns, and, in the case of 
emergency vehicles, sirens. 

Trucks and passenger cars are the major noise-producing vehicles on the nation's highways. 
Motorcycles are also a factor to be considered because of the rapid increase in their numbers in 
recent years. Modem passenger cars are relatively quiet, particularly at the lower cruising speeds, 
but exist in such numbers as to make their total noise contribution significant. While noise 



45 



AASHTO — Geometric Design of Highways and Streets 



produced by passenger cars increases dramatically with speed, steep grades have little influence 
on passenger car noise. 

For passenger cars, noise produced under normal operating conditions is primarily from the 
engine exhaust system and the tire-roadway interaction. Constant highway speeds provide much 
the same noise reading whether or not a car's engine is operating, because the noise is principally 
produced by the tire-roadway interaction with some added wind noise. For conditions of 
maximum acceleration, the engine system noise is predominant. 

Trucks, particularly heavy diesel-powered trucks, present the most difficult noise problem on 
the highway, with the development of more powerful engines generally increasing noise. Truck 
noise levels are not greatly influenced by speed, however, because the factors (including 
acceleration noise) that are not directly affected by speed usually make up a major portion of the 
total noise. In contrast, steep grades can cause an increase in noise levels for large trucks. 

Truck noise has several principal components originating from such sources as exhaust, 
engine gears, fans, and air intake. At higher speeds, tire-roadway interaction and wind noise add 
to the problem. As in passenger cars, the noise produced by large diesel trucks is primarily from 
the engine exhaust system and the tire-roadway interaction. For trucks, however, engine exhaust 
noise tends to dominate tire-roadway interaction for most operating conditions, particularly 
during acceleration. 

The quality of noise varies with the number and operating conditions of the vehicles while 
the directionality and amplitude of the noise vary with highway design features. The highway 
designer should therefore be concerned with how highway location and design influence the 
vehicle noise perceived by persons residing or working nearby. The perceived noise level 
decreases as the distance to the highway from a residence or workplace increases. 



DRIVER PERFORMANCE 
Introduction 

An appreciation of driver performance is essential to proper highway design and operation. 
The suitability of a design rests as much on how safely and efficiently drivers are able to use the 
highway as on any other criterion. When drivers use a highway designed to be compatible with 
their capabilities and limitations, their performance is aided. When a design is incompatible with 
the capabilities of drivers, the chance for driver errors increase, and crashes or inefficient 
operation may result. 

This section provides information about driver performance useful to highway engineers in 
designing and operating highways. It describes drivers in terms of their performance— how they 
interact with the highway and its information system and why they make errors. 

The material draws extensively from A User's Guide to Positive Guidance (4), which 
contains information on driver attributes, driving tasks, and information handling by the driver. 

46 



Design Controls and Criteria 



Where positive guidance is applied to design, competent drivers, using well-designed highways 
with appropriate information displays, can perform safely and efficiently. Properly designed and 
operated highways, in turn, provide positive guidance to drivers. In addition. Transportation 
Research Record 1281 entitled Human Factors and Safety Research Related to Highway Design 
and Operations (5), provides background information. 



Older Drivers 

At the start of the 20th century, approximately 4 percent of America's population was 65 
years of age or older. This group, which accounted for 15 percent of the driving population in 
1986, and is expected to increase to 22 percent by the year 2030. 

Older drivers and pedestrians are a significant and rapidly growing segment of the highway 
user population with a variety of age-related diminished capabilities. As a group, they have the 
potential to adversely affect the highway system's safety and efficiency. There is agreement that 
older road users require mobility and that they should be accommodated by the design and 
operational characteristics of a highway to the extent practical. 

Older drivers have special needs that should be considered in highway design and traffic 
control. For example, for every decade after age 25, drivers need twice the brightness at night to 
receive visual information. Hence, by age 75, some drivers may need 32 times the brightness they 
did at age 25. 

Research findings show that enhancements to the highway system to improve its usability 
for older drivers and pedestrians can also improve the system for all users. Thus, designers and 
engineers should be aware of the capabilities and needs of older road users and consider 
appropriate measures to aid their performance. A Federal Highway Administration report, entitled 
Older Driver Highway Design Handbook: Recommendations and Guidelines (6), provides 
information on how geometric design elements and traffic control devices can be modified to 
better meet the needs and capabilities of older road users. 



The Driving Task 

The driving task depends on drivers receiving and using information correctly. The 
information received by drivers as they travel is compared with the information they already 
possess. Decisions are then made by drivers based on the information available to them and 
appropriate control actions are taken. 

Driving encompasses a number of discrete and interrelated activities. When grouped by 
performance, the components of the driving task fall into three levels: control, guidance, and 
navigation. These activities are ordered on scales of complexity of task and importance for safety. 
Simple steering and speed control are at one end of the scale (control). Road-following and safe- 
path maintenance in response to road and traffic conditions are at midlevel of the scale 
(guidance). At the other end of the scale are trip planning and route following (navigation). 

47 



AASHTO — Geometric Design of Highways and Streets 



The driving task may be complex and demanding, and several individual activities may need 
to be performed simultaneously, requiring smooth and efficient processing and integration of 
information. Driving often occurs at high speeds, under time pressure, in unfamiliar locations, 
and under adverse environmental conditions. The driving task may at other times be so simple 
and undemanding that a driver becomes inattentive. The key to safe, efficient driver performance 
in this broad range of driving situations is error-free information handling. 

Driver errors result from many driver, vehicle, roadway, and traffic factors. Some driver 
errors occur because drivers may not always recognize what particular roadway traffic situations 
are require of them, because situations may lead to task overload or inattentiveness, and because 
deficient or inconsistent designs or information displays may cause confusion. Driver errors may 
also result from pressures of time, complexity of decisions, or profusion of information. Control 
and guidance errors by drivers may also contribute directly to crashes. In addition, navigational 
errors resulting in delay contribute to inefficient operations and may lead indirectly to crashes. 



The Guidance Task 

Of the three major components of the driving task, highway design and traffic operations 
have the greatest effect on guidance. An appreciation of the guidance component of the driving 
task is needed by the highway designer to aid driver performance. 



Lane Placement and Road Following 

Lane placement and road-following decisions, including steering and speed control 
judgments, are basic to vehicle guidance. Drivers use a feedback process to follow alignment and 
grade within the constraints of road and environmental conditions. Obstacle-avoidance decisions 
are integrated into lane placement and road-following activities. This portion of the guidance task 
level is continually performed both when no other traffic is present (singularly) or when it is 
shared with other activities (integrated). 



Car Following 

Car following is the process by which drivers guide their vehicles when following another 
vehicle. Car-following decisions are more complex than road-following decisions because they 
involve speed-control modifications. In car following, drivers need to constantly modify their 
speed to maintain safe gaps between vehicles. To proceed safely, they have to assess the speed of 
the lead vehicle and the speed and position of other vehicles in the traffic stream and continually 
detect, assess, and respond to changes. 



48 



Design Controls and Criteria 



Passing Maneuvers 

The driver decision to initiate, continue, or complete a passing maneuver is even more 
complex than the decisions involved in lane placement or car following. Passing decisions require 
modifications in road- and car-following and in speed control, hi passing, drivers must judge the 
speed and acceleration potential of their own vehicle, the speed of the lead vehicle, the speed and 
rate of closure of the approached vehicle, and the presence of an acceptable gap in the traffic 
stream. 



Other Gyidance Activities 

Other guidance activities include merging, lane changing, avoidance of pedestrians, and 
response to traffic control devices. These activities also require complex decisions, judgments, 
and predictions. 

The Information System 

Each element that provides information to drivers is part of the information system of the 
highway. Formal sources of information are the traffic control devices specifically designed to 
display information to drivers. Informal sources include such elements as roadway and roadside 
design features, pavement joints, tree lines, and traffic. Together, the formal and informal sources 
provide the information drivers need to drive safely and efficiently. Formal and informal sources 
of information are interrelated and must reinforce and augment each other to be most useful. 



Traffic Control Devices 

Traffic control devices provide guidance and navigation information that often is not 
otherwise available or apparent. Such devices include regulatory, warning, and guide signs, and 
other route guidance information. Other traffic control devices, such as markings and delineation, 
display additional information that augments particular roadway or environmental features. These 
devices help drivers perceive information that might otherwise be overlooked or difficult to 
recognize. Information on the appropriate use of traffic control devices is presented in the Manual 
on Uniform Traffic Control Devices (7). 



The Roadway and its Environment 

Selection of speeds and paths is dependent on drivers being able to see the road ahead. 
Drivers must see the road directly in front of their vehicles and far enough in advance to perceive 
with a high degree of accuracy the alignment, profile gradeline, and related aspects of the 
roadway. The view of the road also includes the environment immediately adjacent to the 
roadway. Such appurtenances as shoulders and roadside obstacles (including sign supports, 
bridge piers, abutments, guardrail, and median barriers) affect driving behavior and, therefore, 
should be clearly visible to the driver. 

49 



AASHTO — Geometric Design of Highways and Streets 



Information Handling 

Drivers use many of their senses to gather information. Most information is received visually 
by drivers from their view of the roadway alignment, markings, and signs. However, drivers also 
detect changes in vehicle handling through instinct. They do so, for example, by feeling road 
surface texture through vibrations in the steering wheel and hearing emergency vehicle sirens. 

Throughout the driving task, drivers perform several functions almost simultaneously. They 
look at information sources, make numerous decisions, and perform necessary control actions. 
Sources of information (some needed, others not) compete for their attention. Needed information 
should be in the driver's field of view, available when and where needed, available in a usable 
form, and capable of capturing the driver's attention. 

Because drivers can only attend to one visual information source at a time, they integrate the 
various information inputs and maintain an awareness of the changing environment through an 
attention-sharing process. Drivers sample visual information obtained in short-duration glances, 
shifting their attention from one source to another. They make some decisions immediately, and 
delay others, through reliance on judgment, estimation, and prediction to fill in gaps in available 
information. 



Reaction Tirrie 

Information takes time to process. Drivers' reaction times increase as a function of decision 
complexity and the amount of information to be processed. Furthermore, the longer the reaction 
time, the greater the chance for error. Johannson and Rumar (8) measured brake reaction time for 
expected and unexpected events. Their results show that when an event is expected, reaction time 
averages about 0.6 s, with a few drivers taking as long as 2 s. With unexpected events, reaction 
times increased by 35 percent. Thus, for a simple, unexpected decision and action, some drivers 
may take as long as 2.7 s to respond. A complex decision with several alternatives may take 
several seconds longer than a simple decision. Exhibit 2-26 shows this relationship for median- 
case drivers, whereas Exhibit 2-27 shows this relationship for 85th-percentile drivers. The figures 
quantify the amount of information to be processed in bits. Long processing times decrease the 
time available to attend to other tasks and increase the chance for error. 

Highway designs should take reaction times into account. It should be recognized that 
drivers vary in their responses to particular events and take longer to respond when decisions are 
complex or events are unexpected. Clear sight lines and adequate decision sight distance provide 
a margin for error. 



50 



Design Controls and Criteria 



C 

o 

?: 
a 
o 

# 

m 



5.0 



4M 



3.6 



2.0 



m 













/ 
/ 

/ 
/ 






Unoxf 


/ 
/ 
/ 

/ 
/] 
/ 


/ 




> 


4 
/ 
/ 
/ 
/ 


/ 

/ 


•ctod 




/ 
/ 

// 


/- 

/ 
/ 


/ 








/ 













12 3 4 5 g 

fc^fo^maftof1 Con+<mt (Btf^ 

Exhibit 2-26. Median Driver Reaction Time to Expected and Unexpected Information 



57 



AASHTO — Geometric Design of Highways and Streets 



a.0 



T.O 



6J» 



S.0 



E 



§ 4.0 



0£ 



3,0 



2.0 



1.0 













4 

/ : 










/ 


"f — ^ — : 








/ 






Mn9XQ9* 


Ct«<J /^ 
/ 




y 


y 




/ 

• 


/ 

f 


V^ Exp< 






• 

- .-^ .,/■- - 


/ 

f 










/ 
^ ^ 


y 























2 3 4 

Hformotfon Content CBftsI 



Exhibit 2-27, 85th-PerceiitiIe Driver Reaction Time to Expected and 
Unexpected Information 



52 



Design Controls and Criteria 



Primacy 

Primacy relates the relative importance to safety of competing information. Control and 
guidance information is important because the related errors may contribute directly to crashes. 
Navigation information has a lower primacy because errors may lead to inefficient traffic flow, 
but are less likely to lead to crashes. Accordingly, the design should focus the drivers' attention 
on the safety-critical design elements and high-priority information sources. This goal may be 
achieved by providing clear sight lines and good visual quality. 



Expectancy 

Driver expectancies are formed by the experience and training of drivers. Situations that 
generally occur in the same way, and successful responses to these situations, are incorporated 
into each driver's store of knowledge. Expectancy relates to the likelihood that a driver will 
respond to common situations in predictable ways that the driver has found successful in the past. 
Expectancy affects how drivers perceive and handle information and modify the speed and nature 
of their responses. 

Reinforced expectancies help drivers respond rapidly and correctly. Unusual, unique, or 
uncommon situations that violate driver expectancies may cause longer response times, 
inappropriate responses, or errors. 

Most highway design features are sufficiently similar to create driver expectancies related to 
common geometric, operational, and route characteristics. For example, because most freeway 
interchanges have exits on the right side of the road, drivers generally expect to exit from the 
right. This aids performance by enabling rapid and correct responses when exits on the right are 
to be negotiated. There are, however, instances where expectancies are violated. For example, if 
an exit ramp is on the left, then the right-exit expectancy is incorrect, and response times may be 
lengthened or errors committed. 

One of the most important ways to aid driver performance is to develop designs in 
accordance with prevalent driver expectancies. Unusual design features should be avoided, and 
design elements should be applied consistently throughout a highway segment. Care should also 
be taken to maintain consistency from one segment to another. When drivers obtain the 
information they expect from the highway and its traffic control devices, their performance tends 
to be error free. Where they do not get what they expect, or get what they do not expect, errors 
may result. 



Driver Error 

A common characteristic of many high-crash locations is that they place large or unusual 
demands on the information-processing capabilities of drivers. Inefficient operation and crashes 
usually occur where the driver's chances for information-handling errors are high. At locations 



53 



AASHTO — Geometric Design of Highways and Streets 



where information-processing demands on the driver are high, the possibihty of error and 
inappropriate driver performance increases. 



Errors Due t© Driver Deficiencies 

Many driving errors are caused by deficiencies in a driver's capabilities or temporary states, 
which, in conjunction with inappropriate designs or difficult traffic situations, may produce a 
failure in judgment. For example, insufficient experience and training may contribute to a driver's 
inability to recover from a skid. Similarly, inappropriate risk taking may lead to errors in gap 
acceptance while passing (9). In addition, poor glare recovery may cause older drivers to miss 
information at night (10). 

Adverse psychophysiological states also lead to driver failures. These include decreased 
performance caused by alcohol and drugs, for which a link to crashes has been clearly 
established. The effects of fatigue, caused by sleep deprivation from extended periods of driving 
without rest or prolonged exposure to monotonous environments, or both, also contribute to 

crashes (11). 

It is not generally possible for a design or an operational procedure to reduce errors caused 
by innate driver deficiencies. However, designs should be as forgiving as practical to lessen the 
consequences of such failures. Errors committed by competent drivers can be reduced by proper 
design and operation. Most individuals possess the attributes and skills to drive properly and are 
neither drunk, drugged, nor fatigued at the start of their trips. When drivers overextend 
themselves, fail to take proper rest breaks, or drive for prolonged periods, they ultimately reach a 
less-than-competent state. Fatigued drivers represent a sizable portion of the long-trip driving 
population and should therefore be considered in freeway design. 

Although opinions among experts are not unanimous, there is general agreement that 
advancing age has a deleterious effect on an individual's perceptual, mental, and motor skills. 
These skills are critical factors in vehicular operation. Therefore, it is important for the road 
designer to be aware of the needs of the older driver, and where appropriate, to consider these 
needs in the roadway design. 

Some of the more important information and observations from recent research studies 
concerning older drivers is summarized below: 

1. Characteristics of the Older Driver. In comparison to younger drivers, older drivers 
often exhibit the following operational deficiencies: 

@ slower information processing 

® slower reaction times 

® slower decision making 

® visual deterioration 

® hearing deterioration 

® decline in ability to judge time, speed, and distance 

54 



Design Controls and Criteria 



® limited depth perception 
® limited physical mobility 



® side effects from prescription drugs 

2. Crash Frequency. Older drivers are involved in a disproportionate number of crashes 
where there is a higher-than-average demand imposed on driving skills. The driving 
maneuvers that most often precipitate higher crash frequencies among older drivers 
include: 

® making left turns across traffic 

® merging with high-speed traffic 

® changing lanes on congested streets in order to make a turn 

® crossing a high-volume intersection 

® stopping quickly for queued traffic 

® parking 

3. Coiintermeasiires. The following countermeasures may help to alleviate the potential 
problems of the older driver: 

® assess all guidelines to consider the practicality of designing for the 95th- or 99th- 
percentile driver, as appropriate, to represent the performance abilities of an older 
driver 

® improve sight distance by modifying designs and removing obstructions, 
particularly at intersections and interchanges 

® assess sight triangles for adequacy of sight distance 

® provide decision sight distances 

® simplify and redesign intersections and interchanges that require multiple 
information reception and processing 

® consider alternate designs to reduce conflicts 

® increase use of protected left-turn signal phases 

® increase vehicular clearance times at signalized intersections 

^ provide increased walk times for pedestrians 

® provide wider and brighter pavement markings 

^ provide larger and brighter signs 

® reduce sign clutter 

® provide more redundant information such as advance guide signs for street name, 
indications of upcoming turn lanes, and right-angle arrows ahead of an intersection 
where a route turns or where directional information is needed 

® enforce speed limits 

® increase driver education 

In roadway design, perhaps the most practical measure related to better accommodate older 
drivers is an increase in sight distance, which may be accomplished through increased use of 
decision sight distance. The gradual aging of the driver population suggests that increased use of 
decision sight distance may help to reduce future crash frequencies for older drivers. Where 



55 



AASHTO — Geometric Design of Highways and Streets 



provision of decision sight distance is impractical, increased use of advance warning or guide 
signs may be appropriate. 



Errors Due to Situation Demands 

Drivers often commit errors when they have to perform several highly complex tasks 
simultaneously under extreme time pressure (12). Errors of this type usually occur at urban 
locations with closely spaced decision points, intensive land use, complex design features, and 
heavy traffic. Information-processing demands beyond the drivers' capabilities may cause 
information overload or confuse drivers, resulting in an inadequate understanding of the driving 
situation. 

Other locations present the opposite situations and are associated with different types of 
driver errors. Typically these are rural locations where there may be widely spaced decision 
points, sparse land use, smooth alignment, and light traffic. Information demands are thus 
minimal, and rather than being overloaded with information, the lack of information and 
decision-making demands may result in inattentiveness by drivers. Driving errors may be caused 
by a state of decreased vigilance in which drivers fail to detect, recognize, or respond to new, 
infrequently encountered, or unexpected design elements or information sources. 



Speed and Design 

Speed reduces the visual field, restricts peripheral vision, and limits the time available for 
drivers to receive and process information. Highways built to acconmiodate high speeds help 
compensate for these limitations by simplifying control and guidance activities, by aiding drivers 
with appropriate information, by placing this information within the cone of clear vision, by 
eliminating much of the need for peripheral vision, and by simplifying the decisions required and 
spacing them farther apart to decrease information-processing demands. 

Current freeway designs have nearly reached the goal of allowing drivers to operate at high 
speeds in comfort and safety. Control of access to the traveled way reduces the potential for 
conflicts by giving drivers a clear path. Clear roadsides have been provided by eliminating 
obstructions or designing them to be more forgiving. The modem freeway provides an alignment 
and profile that, together with other factors, encourages high operating speeds. 

Although improved design has produced significant benefits, it has also created potential 
problems. For example, driving at night at high speeds may lead to reduced forward vision 
because of the inability of headlights to illuminate objects in the driver's path in sufficient time 
for some drivers to respond (13). In addition, the severity of crashes is generally greater with 
increased speed. 

Finally, the very fact that freeways succeed in providing safe, efficient transportation can 
lead to difficulties. The Institute of Traffic Engineers (14) indicated that "Freeways encourage 



56 



Design Controls and Criteria 



drivers to extend the customary length and duration of their trips. This results in driver fatigue 
and slower reaction as well as a reduction in attention and vigilance." 

Thus, extended periods of high-speed driving on highways with low demand for information 
processing may not always be conducive to proper information handling by drivers and may 
therefore lead to driver fatigue. Highway design should take these possible adverse effects into 
account and seek to lessen their consequences. For example, long sections of flat, tangent 
roadway should be avoided and flat, curving alignment that follows the natural contours of the 
terrain should be used whenever practical. Rest areas spaced at intervals of approximately one 
hour or less of driving time have also proved beneficial. 



Design Assessment 

The preceding sections of this chapter have described the way drivers use information 
provided by the highway and its appurtenances. This discussion has shown the interdependence 
between design and information display. Both should be assessed in the design of highway 
projects. Because drivers 'Vead" the road and the adjacent environment and make decisions based 
on what they see (even if traffic control devices making up the formal information system 
indicate inconsistencies with the driver's view), a highway segment that is inappropriately 
designed may not operate safely and efficiendy. Conversely, an adequately designed highway 
may not operate properly without the appropriate complement of traffic control devices. 

Designers should consider how the highway will fit into the existing landscape, how the 
highway should be signed, and the extent to which the information system will complement and 
augment the proposed design. The view of the road is very important, especially to the unfamiliar 
driver. Therefore, consideration should be given to the visual qualities of the road. This can be 
accomplished through the use of 3-D computer visualization programs. 

Locations with potential for information overload should be identified and corrected. The 
adequacy of the sight lines and sight distances should be assessed, and it should be determined 
whether unusual vehicle maneuvers are required and whether likely driver expectancies may be 
violated. 

Potential driver problems can be anticipated before a facility is built by using information 
about the driving tasks and possible driver errors to assess the design. When trade-offs are 
appropriate, they should be made with the drivers' capabilities in mind to ensure that the resultant 
design is compatible with those capabilities. Properly designed highways that provide positive 
guidance to drivers can operate at a high level of safety and efficiency; therefore, designers 
should seek to incorporate these principles in highway design. 



57 



AASHTO — Geometric Design of Highways and Streets 



TRAFFIC CHARACTERISTICS 
General Consideratioris 

The design of a highway and its features should be based upon explicit consideration of the 
traffic volumes and characteristics to be served. All information should be considered jointly. 
Financing, quality of foundations, availability of materials, cost of right-of-way, and other factors all 
have important bearing on the design; however, traffic volumes indicate the need for the improvement 
and directly affect the geometric design features, such as number of lanes, widths, alignments, and 
grades. It is no more rational to design a highway without traffic information than it is to design a 
bridge without knowledge of the weights and numbers of vehicles it is intended to support. 
Information on traffic volumes serves to establish the loads for the geometric highway design. 

Traffic data for a road or section of road are generally available or can be obtained from field 
studies. The data collected by State or local agencies include traffic volumes for days of the year and 
time of the day, as well as the distribution of vehicles by type and weight. The data also include 
information on trends from which the designer may estimate the traffic to be expected in the future. 

Volume 

Average Daily Traffic 

The most basic measure of the traffic demand for a highway is the average daily traffic (ADT) 
volume. The ADT is defined as the total volume during a given time period (in whole days), greater 
than one day and less than one year, divided by the number of days in that time period. The current 
ADT volume for a highway can be readily determined when continuous traffic counts are available. 
When only periodic counts are taken, the ADT volume can be estimated by adjusting the periodic 
counts according to such factors as the season, month, or day of week. 

Knowledge of the ADT volume is important for many purposes, such as determining annual 
highway usage as justification for proposed expenditures or designing the structural elements of a 
highway. However, the direct use of ADT volume in the geometric design of highways is not 
appropriate except for local and collector roads with relatively low volumes because it does not 
indicate traffic volume variations occurring during the various months of the year, days of the week, 
and hours of the day. The amount by which the volume of an average day is exceeded on certain days 
is appreciable and varied. At typical rural locations, the volume on certain days may be significantly 
higher than the ADT. Thus, a highway designed for the traffic on an average day would be required to 
carry a volume greater than the design volume for a considerable portion of the year, and on many 
days the volume carried would be much greater than the design volume. 



58 



Design Controls and Criteria 



Peak-Hour Traffic 

Traffic volumes for an interval of time shorter than a day more appropriately reflect the 
operating conditions that should be used for design. The brief, but frequently repeated, rush-hour 
periods are significant in this regard. In nearly all cases, a practical and adequate time period is 
one hour. 

The traffic pattern on any highway shows considerable variation in traffic volumes during 
the various hours of the day and in hourly volumes throughout the year. A key design decision 
involves determining which of these hourly traffic volumes should be used as the basis for design. 
While it would be wasteful to predicate the design on the maximum peak-hour traffic that occurs 
during the year, the use of the average hourly traffic would result in an inadequate design. The 
hourly traffic volume used in design should not be exceeded very often or by very much. On the 
other hand, it should not be so high that traffic would rarely be sufficient to make full use of the 
resulting facility. One guide in determining the hourly traffic volume that is best suited for use in 
design is a curve showing variation in hourly traffic volumes during the year. 

Exhibit 2-28 shows the relationship between the highest hourly volumes and ADT on rural 
arterials. This figure was produced from an analysis of traffic count data covering a wide range of 
volumes and geographic conditions. The curves in the chart were prepared by arranging all of the 
hourly volumes for one year, expressed as a percentage of ADT, in a descending order of 
magnitude. The middle curve is the average for all locations studied and represents a highway 
with average fluctuation in traffic flow. 

Based on a review of these curves, it is recommended that the hourly traffic volume that 
should generally be used in design is the 30th highest hourly volume of the year, abbreviated as 
30 HV. The reasonableness of 30 HV as a design control is indicated by the changes that result 
from choosing a somewhat higher or lower volume. The curve in Exhibit 2-28 steepens quickly to 
the left of the point showing the 30th highest hour volume and indicates only a few more hours 
with higher volumes. The curve flattens to the right of the 30th highest hour and indicates many 
hours in which the volume is not much less than the 30 HV. 

On rural roads with average fluctuation in traffic flow, the 30 HV is typically about 15 
percent of the ADT. Whether or not this hourly volume is too low to be appropriate for design 
can be judged by the 29 hours during the year when it is exceeded. The maximum hourly volume, 
which is approximately 25 percent of the ADT on the graph, exceeds 30 HV by about 67 percent. 

Whether the 30 HV is too high for practical economy in design can be judged by the trend in 
the hourly volumes lower than the 30th highest hour. The middle curve in Exhibit 2-28 indicates 
that the traffic volume exceeds 11.5 percent of the ADT during 170 hours of the year. The lowest 
of this range of hourly volumes is about 23 percent less than the 30 HV. 



59 



AASHTO — Geometric Design of Highways and Streets 



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Oreof^r ihon fhiaf Shown 



Exhibit 2-28. Relation Between Peak-Hour and Average Daily Traffic Volumes on 

Rural Arterials 



Another fortunate characteristic of 30 HV is that, as a percentage of ADT, it generally varies 
only slightly from year to year even though the ADT may change substantially. Increased ADT 
generally results in a slight decrease in the percentage of ADT during the 30 HV. Thus, the 
percentage of ADT used for determining the 30 HV from current traffic data for a given facility can 
generally be used with confidence in computing the 30 HV from an ADT volume determined for 
some future year. This consistency between current and future may not apply where there is a radical 
change in the use of the land area served by the highway. In cases where the character and magnitude 
of future development can be foreseen, the relationship of 30 HV to ADT may be based on 
experience with other highways serving areas with similar land-use characteristics. 

For highway design purposes, the variation in hourly traffic volumes should be measured and the 
percentage of ADT during the 30th highest hour determined. Where such measurements are 
impractical and only the ADT is known, the 30 HV should be estimated from 30th-hour percentage 
factors for similar highways in the same locality, operating under similar conditions. 



60 



Design Controls and Criteria 



On a typical rural arterial, the 30 HV is about 15 percent of ADT, and the maximum hourly 
volume is about 25 percent of ADT. As indicated in Exhibit 2-28, the 30 HV at 70 percent of all 
locations, except those having unusually high or low fluctuation in traffic flow, is in the range of 
12 to 18 percent of the ADT. Likewise the range in maximum hourly volumes for the same 
groups of roads varies approximately from 16 to 32 percent of the ADT. These criteria for design 
apply to most rural highways. There are highways, however, for which there are unusual or 
highly seasonal fluctuations in traffic flow, such as resort roads on which weekend traffic during 
a few months of the year far exceeds the traffic during the rest of the year. Seasonal fluctuations 
result in high peak-hour volumes relative to ADT, high percentages for high-volume hours, and 
low percentages for low-volume hours. 

Because the percentage represented by the 30 HV for a road with large seasonal fluctuations 
may not be much different from the percentage represented by the 30 HV on most rural roads, the 
30 HV criterion may not be appropriate for such roads. A design that results in somewhat less 
satisfactory traffic operation during seasonal peaks than on rural roads with normal traffic 
fluctuations, will generally be accepted by the public. On the other hand, design should not be so 
economical that severe congestion results during peak hours. It may be desirable, therefore, to 
choose an hourly volume for design, which is about 50 percent of the volumes expected to occur 
during a few highest hours of the design year, whether or not that volume is equal to 30 HV. 
Some congestion would be experienced by traffic during peak hours but the capacity would not 
be exceeded. A check should be made to ensure that the expected maximum hourly traffic does 
not exceed the capacity. 

The design hourly volume (DHV) for rural highways, therefore, should generally be the 30 
HV of the future year chosen for design. Exceptions may be made on roads with high seasonal 
traffic fluctuation, where a different hourly volume may need to be used. The 30-HV criterion 
also applies in general to urban areas; however, where the fluctuation in traffic flow is markedly 
different from that on rural highways, other hours of the year should be considered as the basis 
for design. 

In urban areas, an appropriate DHV may be determined from the study of traffic during the 
normal daily peak periods. Because of the recurring morning and afternoon peak traffic flow, 
there is usually little difference between the 30th and the 200th highest hourly volume. For 
typical urban conditions, the highest hourly volume is found during the afternoon work-to-home 
travel peak. One approach for determining a suitable DHV is to select the highest afternoon peak 
traffic flow for each week and then average these values for the 52 weeks of the year. If the 
morning peak-hour volumes for each week of the year are all less than the afternoon peak 
volumes, the average of the 52 weekly afternoon peak-hour volumes would have about the same 
value as the 26th highest hourly volume of the year. If the morning peaks are equal to the 
afternoon peaks, the average of the afternoon peaks would be about equal to the 50th highest 
hourly volume. 

The volumes represented by the 26th and 50th highest hours of the year are not sufficienfly 
different from the 30 HV value to affect design. Therefore, in urban design, the 30th highest 
hourly volume can also be assumed to be a reasonable representation of daily peak hours during 
the year. Exceptions may be appropriate in those areas or locations where recreational or other 

61 



AASHTO — Geometric Design of Highways and Streets 



travel is concentrated during particular seasons. At such locations, a distribution of traffic volume 
where the hourly volumes are much greater than the 30 HV may result; the 30 HV in such cases 
may be inappropriate as the DHV and a higher value should be considered in design. Specific 
measurements of traffic volumes should be made and evaluated to determine the appropriate 

DHV. 

Traffic estimates used for the design of urban streets and highways are usually expressed as 
ADT volumes derived from the urban transportation planning process. In recent years, however, 
consideration has been given to the development of DHVs by making peak-hour traffic 
assignments in lieu of ADT assignments. The availability of the 1980 and 1990 census joumey- 
to-work information has had a major influence on this latter approach. 

In the usual case, future travel demand is determined from the urban transportation planning 
process in terms of total daily trips that are assigned to the transportation system. Consideration 
of the split between public and private transportation is also incorporated into this process. These 
assigned trips constitute the traffic volumes on links of the future street and highway network. 

In some instances, these volumes (ADT) are provided directly to highway designers. In 
others, they are converted by the operational transportation study staff to directional volumes for 
the design hour. From a practical standpoint, the latter approach may be the more desirable 
because the transportation study staff is often in a better position to evaluate the effects that the 
assumptions inherent in the planning process have on the resulting design volumes. 

Two-way DHVs (i.e., the 30 HV, or its equivalent) may be determined by applying a 
representative percentage (usually 8 to 12 percent in urban areas) to the ADT. In many cases this 
percentage, based on data obtained in a traffic count program, is developed and applied system- 
wide; in other cases, factors may be developed for different facility classes or different areas of an 
urban region, or both. At least one highway agency has developed regression equations 
representing the relationship between peak flow and ADT; different equations are applied, 
depending on the number of lanes and the range of the ADT volumes. 



Directional Distribution 

For two-lane rural highways, the DHV is the total traffic in both directions of travel. In the 
design of highways with more than two lanes and on two-lane roads where important 
intersections are encountered or where additional lanes are to be provided later, knowledge of the 
hourly traffic volume for each direction of travel is essential. 

A multilane highway with a high percentage of traffic in one direction during the peak hours 
may need more lanes than a highway having the same ADT but with a lesser percentage of 
directional traffic. During peak hours on most rural highways, from 55 to 70 percent of the traffic 
is traveling in the peak direction, with up to as much as 80 percent occasionally. Directional 
distributions of traffic vary enough between sites that two multilane highways carrying equal 
traffic may have peak direction volumes that differ by as much as 60 percent. For example, 
consider a rural road with a design volume of 4,000 vehicles per hour (vph) for both directions of 

62 



Design Controls and Criteria 



travel combined. If during the design hour, the directional distribution is equally split, or 2,000 vph is 
one direction, two lanes in each direction may be adequate. If 80 percent of the DHV is in one 
direction, at least three lanes in each direction would be needed for the 3,200 vph; and if a 
1,000-vehicles-per-lane criterion is appUed, four lanes in each direction would be needed. 

The peak-hour traffic distribution by direction of travel is generally consistent from day to day 
and from year to year on a given rural road, except on some highways serving recreational areas. 
Except for urban highways, the directional distribution of traffic measured for current conditions may 
generally be assumed to apply to the DHV for the future year for which the facility is designed. 

The directional distribution of traffic on multilane facilities during the design hour (DDHV) 
should be determined by making field measurements on the facility under consideration or on parallel 
and similar facilities. In the latter case, the parallel facilities should preferably be those from which 
traffic, for the most part, would be diverted to the new highway. The DDHV applicable for use on 
multilane facilities may be computed by multiplying the ADT by the percentage that 30 HV is of the 
ADT, and then by the percentage of traffic in the peak direction during the design hour. Thus, if the 
DHV is 15 percent of the ADT and the directional distribution at the peak hour is 60:40, the DDHV is 
0.15 X 0.60 X ADT, or 9 percent of the ADT. If the directional ADT is known for only one direction, 
the ADT is nearly always twice the directional ADT. 

In designing intersections and interchanges, the volumes of all movements occurring during the 
design hour should be known. This information is needed for both the morning and evening peak 
periods because the traffic pattern may change significantly from one peak hour to the other. 
Normally, a design is based on the DHV, which is to be accommodated during the morning rush hour 
in one direction and during the evening rush hour in the other direction. Total (two-way) volumes 
may be the same during both of these peaks, but the percentage of traffic in the two directions of 
travel is reversed. At intersections, the percentage of approaching traffic that turns to the right and to 
the left on each intersection leg should be determined separately for the morning and evening peak 
periods. This information should be determined from actual counts, from origin and destination data, 
or both. 



Composition of Traffic 

Vehicles of different sizes and weights have different operafing characteristics that should be 
considered in highway design. Besides being heavier, trucks are generally slower and occupy more 
roadway space. Consequently, trucks have a greater individual effect on highway traffic operation 
than do passenger vehicles. The effect on traffic operation of one truck is often equivalent to several 
passenger cars. The number of equivalent passenger cars equaling the effect of one truck is dependent 
on the roadway gradient and, for two-lane highways, on the available passing sight distance. Thus, the 
larger the proportion of trucks in a traffic stream, the greater the equivalent traffic demand and the 
greater the highway capacity needed. 



63 



AASHTO — Geometric Design of Highways and Streets 



For uninterrupted traffic flow, as typically found in rural areas, the various sizes and weights of 
vehicles, as they affect traffic operation, can be grouped into two general classes: 

® Passenger cars — all passenger cars, including mini-vans, vans, pick-up trucks, and 

sport/utility vehicles 
© Trucks — all buses, single-unit trucks, combination trucks, and recreation vehicles 

For traffic-classification purposes, trucks are normally defined as those vehicles having 
manufacturer's gross vehicle weight (GVW) ratings of 4,000 kg [9,000 lb] or more and having dual 
tires on at least one rear axle. 

hi the passenger-car class, as defined above, most of the vehicles have similar operating 
characteristics. In the truck class, operating characteristics vary considerably, particularly in size and 
weight/power ratio. Despite this variation in the operating characteristics of trucks, the average effect 
of all trucks in a traffic stream is similar on most highways under comparable conditions. 
Accordingly, for the geometric design of a highway, it is essential to have traffic data on vehicles in 
the truck class. These data generally indicate the major types of trucks and buses as percentages of all 
traffic expected to use the highway. 

For design purposes, the percentage of truck traffic during the peak hours should be determined. 
In rural areas, comprehensive data usually are not available on the distribution of traffic by vehicle 
types during the peak hours; however, the percentage of truck traffic during the peak hours is 
generally less than the percentage for a 24-hour period. As the peak hour approaches, the volume of 
passenger-car traffic generally increases at a greater rate than does the volume of truck traffic. Most 
trucks operate steadily throughout the day, and much over-the-road hauling is done at night and 
during early morning hours. In the vicinity of major truck and bus terminals, the scheduling of regular 
truck and bus runs may result in the concentration of trucks during certain hours of the day. However, 
because of the delays caused by other traffic during peak hours, such schedules generally are made to 
avoid these hours. 

For design of a particular highway, data on traffic composition should be determined by traffic 
studies. Truck traffic should be expressed as a percentage of total traffic during the design hour (in 
the case of a two-lane highway, as a percentage of total two-way traffic, and in the case of a multilane 
highway, as a percentage of total traffic in the peak direction of travel). 

Under urban interrupted-flow conditions, the criteria for determining traffic composition differ 
from those used elsewhere. At important intersections, the percentage of trucks during the morning 
and evening peak hours should be determined separately. Variations in truck traffic between the 
various traffic movements at intersections may be substantial and may influence the appropriate 
geometric layout. The percentage of trucks may also vary considerably during a particular hour of the 
day. Therefore, it is advisable to count trucks for the several peak hours that are considered 
representative of the 30th highest or design hour. A convenient value, that appears appropriate for 
design use, is the average of the percentages of truck traffic percentages for a number of weekly peak 
hours. For highway-capacity analysis purposes, local city-transit buses should be considered 
separately from other trucks and buses. 

64 



Design Controls and Criteria 



Projection of Future Traffic Demands 

Geometric design of new highways or improvements to existing highways should not usually be 
based on current traffic volumes alone, but should consider future traffic volumes expected to use the 
facility. A highway should be designed to accommodate the traffic volume that is likely to occur 
within the design life of the facility. 

It is difficult to define the life of a highway because major segments may have different lengths 
of physical life. Each segment is subject to variations in estimated life expectancy for reasons not 
readily subject to analysis, such as obsolescence or unexpected radical changes in land use, with the 
resulting changes in traffic volumes, patterns, and demands. Right-of-way and grading may be 
considered to have a physical life expectancy of 100 years; minor drainage structures and base 
courses, 50 years; bridges, 25 to 100 years; resurfacing, 10 years; and pavement structure, 20 to 30 
years, assuming adequate maintenance and no allowance for obsolescence. Bridge life may vary 
depending on the cumulative frequency of heavy loads. Pavement life can vary widely, depending 
largely on initial expenditures and the repetition of heavy axle loads. 

The assumption of no allowance for functional obsolescence is open to serious debate. The 
principal causes of obsolescence are increases in the number of intersections and driveways, and 
increases in traffic demand beyond the design capacity. On non-freeway highways, obsolescence due 
to addition of intersections and driveways is much more difficult to forestall; this occurs particularly 
in urban and suburban areas, but may occur in rural areas as well. 

It is a moot question whether the design capacity of a highway should be based on its life 
expectancy. The decision is greatly influenced by economics. For example, a highway might be 
designed for traffic volumes 50 years hence with the expectation that the pavement structure would be 
restored in 20 to 25 years. However, if the added cost of a 50-year design over a design with a 25- 
year life expectancy is appreciable, it may be imprudent to make a further investment providing 
capacity that will not be needed for at least 25 years. The construction cost savings could be used to 
construct another currently needed highway project. Furthermore, the cost of increased maintenance 
for the larger highway would be avoided for at least 25 years. Also, most highways are capable of 
handling higher traffic volumes than their design volume indicates, but this may cause more 
inconvenience, such as a reduction in speed and less maneuverability. 

For example, a four-lane divided highway with a design ADT of 10,000 or 15,000 vehicles per 
day could handle two or three times that design volume depending on several factors discussed later. 
Thus, the four-lane divided highway could adequately serve traffic long after the design year and, in 
many cases, indefinitely. 

In a practical sense, the design volume should be a value that can be estimated with reasonable 
accuracy. Many highway engineers believe the maximum design period is in the range of 15 
to 24 years. Therefore, a period of 20 years is widely used as a basis for design. Traffic 



65 



AASHTO — Geometric Design of Highways and Streets 



cannot usually be forecast accurately beyond this period on a specific facility because of probable 
changes in the general regional economy, population, and land development along the highway, 
which cannot be predicted with any degree of assurance. 

Estimating traffic volumes for a 20-year design period may not be appropriate for many 
reconstruction or rehabilitation projects. These projects may be developed on the basis of a shorter 
design period (5 to 10 years) because of the uncertainties of predicting traffic and funding constraints. 



Speed 

Speed is one of the most important factors considered by travelers in selecting alternative routes 
or transportation modes. Travelers assess the value of a transportation facility in moving people and 
goods by its convenience and economy, which are directly related to its speed. The attractiveness of a 
public transportation system or a new highway are each weighed by the travelers in terms of time, 
convenience, and money saved. Hence, the desirability of rapid transit may well rest with how rapid it 
actually is. The speed of vehicles on a road or highway depends, in addition to capabilities of the 
drivers and their vehicles, upon five general conditions: the physical characteristics of the highway, 
the amount of roadside interference, the weather, the presence of other vehicles, and the speed 
limitations (established either by law or by traffic control devices). Although any one of these factors 
may govern travel speed, the effect of these general conditions is usually interrelated. 

The objective in design of any engineered facility used by the public is to satisfy the public's 
demand for service in a safe and economical manner. The facility should, therefore, accommodate 
nearly all demands with reasonable adequacy and also should not fail under severe or extreme traffic 
demands. Therefore, highways should be designed to operate at a speed that satisfies nearly all 
drivers. Because only a small percentage of drivers travel at extremely high speed, it is not 
economically practical to design for them. They can use the highway, of course, but will be 
constrained to travel at speeds less than they consider desirable. On the other hand, the speed chosen 
for design should not be that used by drivers under unfavorable conditions, such as inclement 
weather, because the highway would then be inefficient, and possibly unsafe, for drivers under 
favorable conditions, and would not satisfy reasonable public expectations for the facility. 



Operating Speed 

Operating speed is the speed at which drivers are observed operating their vehicles during free- 
flow conditions. The 85th percentile of the distribution of observed speeds is the most frequently used 
measure of the operating speed associated with a particular location or geometric feature. 



66 



Design Controls and Criteria 



Ryoriing Speed 

The speed at which an individual vehicle travels over a highway section is known as its 
running speed. The running speed is the length of the highway section divided by the running 
time required for the vehicle to travel through the section. The average running speed of all 
vehicles is the most appropriate speed measure for evaluating level of service and road user costs. 
The average running speed is the sum of the distances traveled by vehicles on a highway section 
during a specified time period divided by the sum of their running times. 

One means of estimating the average running speed for an existing facility where flow is 
reasonably continuous is to measure the spot speed at one or more locations. The average spot 
speed is the arithmetic mean of the speeds of all traffic as measured at a specified point on the 
roadway. For short sections of highway, on which speeds do not vary materially, the average spot 
speed at one location may be considered an approximation of the average running speed. On 
longer stretches of rural highway, average spot speeds measured at several points, where each 
point represents the speed characteristics of a selected segment of highway, may be averaged 
(taking relative lengths of the highway segments into account) to provide a better approximation 
of the average running speed. 

The average running speed on a given highway varies somewhat during the day, depending 
primarily on the traffic volume. Therefore, when reference is made to a running speed, it should 
be clearly stated whether this speed represents peak hours, off-peak hours, or an average for the 
day. Peak and off-peak running speeds are used in design and operation; average running speeds 
for an entire day are used in economic analyses. 

The effect of traffic volume on average running speed can be determined using the 
procedures of the Highway Capacity Manual (HCM) (15). The HCM shows that: 

^ for freeways and multilane highways, there is a substantial range of flow rates over 
which speed is relatively insensitive to the flow rate; this range extends to fairly high 
flow rates. Then, as the flow rate per lane approaches capacity, speed decreases 
substantially with increasing flow rate. 

© for two-lane highways, speed decreases linearly with increasing flow rate over the 
entire range of flow rates between zero and capacity. 



Design Speed 

Design speed is a selected speed used to determine the various geometric design features of 
the roadway. The assumed design speed should be a logical one with respect to the topography, 
anticipated operating speed, the adjacent land use, and the functional classification of highway. 
Except for local streets where speed controls are frequently included intentionally, every effort 
should be made to use as high a design speed as practical to attain a desired degree of safety, 
mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, 
and social or political impacts. Once the design speed is selected, all of the pertinent highway 
features should be related to it to obtain a balanced design. Above-minimum design values should 

67 



AASHTO — Geometric Design of Highways and Streets 



be used, where practical. Some design features, such as curvature, superelevation, and sight 
distance, are directly related to, and vary appreciably with, design speed. Other features, such as 
widths of lanes and shoulders and clearances to walls and rails, are not directly related to design 
speed, but they do affect vehicle speeds. Therefore, wider lanes, shoulders, and clearances should 
be considered for higher design speeds. Thus, when a change is made in design speed, many 
elements of the highway design will change accordingly. 

The selected design speed should be consistent with the speeds that drivers are likely to 
expect on a given highway facility. Where a reason for limiting speed is obvious, drivers are more 
apt to accept lower speed operation than where there is no apparent reason. A highway of higher 
functional classification may justify a higher design speed than a lesser classified facility in 
similar topography, particularly where the savings in vehicle operation and other operating costs 
are sufficient to offset the increased costs of right-of-way and construction. A low design speed, 
however, should not be selected where the topography is such that drivers are likely to travel at 
high speeds. Drivers do not adjust their speeds to the importance of the highway, but to their 
perception of the physical limitations of the highway and its traffic. 

The selected design speed should fit the travel desires and habits of nearly all drivers 
expected to use a particular facility. Where traffic and roadway conditions are such that drivers 
can travel at their desired speed, there is always a wide range in the speeds at which various 
individuals will choose to operate their vehicles. A cumulative distribution of free-flow vehicle 
speeds typically has an S-shape when plotted as the percentage of vehicles versus observed speed. 
The selected design speed should be a high-percentile value in this speed distribution curve (i,e., 
inclusive of nearly all of the desired speeds of drivers, wherever practical). 

Speed distribution curves illustrate the range of speeds that should be considered in selecting 
an appropriate design speed. A design speed of 110 km/h [70 mph] should be used for freeways, 
expressways, and other arterial highways in rural areas. 

It is desirable that the running speed of a large proportion of drivers be lower than the design 
speed. Experience indicates that deviations from this desired goal are most evident and 
problematic on sharper horizontal curves. In particular, curves with low design speeds (relative to 
driver expectation) are frequently overdriven and tend to have poor safety records. Therefore, it is 
important that the design speed used for horizontal curve design be a conservative reflection of 
the expected speed on the constructed facility. 

Where the physical features of the highway are the principal speed controls and where most 
drivers choose to operate near the speed limit, a design speed of 120 km/h [75 mph] would serve 
a very high percentage of drivers. On a highway designed for this speed, only a small percentage 
of drivers might operate at higher speeds when volume is low and all other conditions are 
favorable. However, for a design speed of 80 km/h [50 mph], satisfactory performance could be 
expected only on certain types of highways. When a low speed design is selected, it may be 
important to have the speed limit enforced during off-peak hours. 

On many freeways, particularly in suburban and rural areas, a design speed of 100 km/h 
[60 mph] or higher can be provided with little additional cost above that required for a design 

68 



Design Controls and Criteria 



Speed of 80 km/h [50 mph]. If the freeway alignment is relatively straight and the character and 
location of interchanges permit design for high-speed operation, a design speed of 1 10 km/h [70 
mph] is desirable. 

Generally, there is no distinction in design speed between ground-level, elevated, and 
depressed freeways. However, the operating characteristics of elevated freeways differ from those 
on depressed freeways. On a depressed freeway, traffic exits the freeway on upgrade ramps and 
enters the freeway on downgrade ramps, which encourages good operation. By contrast, on an 
elevated freeway, traffic exits the freeway on downgrade ramps and enters the freeway on 
upgrade ramps, which is less desirable because vehicles entering the elevated freeway on an 
ascending grade, particularly loaded trucks, require long distances to reach the running speed of 
the freeway. Furthermore, vehicles leaving the elevated freeway on a descending grade need 
additional braking distance before reaching the arterial street, and therefore, may tend to slow 
down in the through-traffic lanes in advance of the ramp terminal. Parallel deceleration lanes or 
longer ramp lengths and lesser grades are frequently used on elevated freeways to reduce the 
likelihood that vehicles will slow in the main lanes. Nevertheless, running speeds on elevated 
freeways are apt to be slightly lower than those on similar depressed freeways, especially when 
access points are closely spaced. In northern climates, elevated structures are subject to rapid 
freezing of precipitation as a result of their exposure; the use of lower superelevation rates may be 
appropriate under such conditions. Although speeds on viaducts are less than those on 
comparable depressed sections, the difference probably is small. Therefore, design speeds of 80 
to 1 10 km/h [50 to 70 mph] apply to both elevated and depressed freeways. 

Given an overall range in design speeds of 20 to 120 knn/h [15 to 75 mph] used in geometric 
design, it is desirable to select design speeds in increments of 10 km/h [5 mph]. Smaller 
increments would result in litde distinction in the dimensions of design elements between one 
design speed and the next higher design speed; larger increments of 20 to 30 km/h [15 to 20 mph] 
would result in too large a difference in the dimensions of design features between any two 
design speeds. In some instances, however, there may be an advantage in using intermediate 
increments to effect changes in the design speed. Increments in design speed of 10 km/h [5 mph] 
should also be used in the design of turning roadways, ramps, and low-speed roads. 

Exhibit 2-29 shows the corresponding design speeds in metric and U.S. customary units in 
10-km/h [5-mph] increments. This table should be used in converting the units of measurement of 
design speeds. 

Although the selected design speed establishes the Umiting values of curve radius and 
minimum sight distance that should be used in design, there should be no restriction on the use of 
flatter horizontal curves or greater sight distances where such improvements can be provided as a 
part of an economical design. Even in rugged terrain, an occasional tangent or flat curve may be 
desirable. Isolated features designed for higher speeds would not necessarily encourage drivers to 
speed up, although a succession of such features might. In such cases, the entire section of 
highway should be designed for a higher speed. A substantial length of tangent between sections 
of curved alignment is also likely to encourage high-speed operation. In such situations, a higher 
design speed should be selected for all geometric features, particularly sight distance on crest 
vertical curves and across the inside of horizontal curves. 

69 



AASHTO — Geometric Design of Highways and Streets 



Metric 


US Customary 


Design speed (km/h) 


Corresponding design speed (mph) 


20 


15 i 


30 


20 


40 


25 


50 


30 


60 


40 


70 


45 


80 


50 


90 


55 


100 


60 


110 


70 


120 ' 


75 


130 


80 



Exhibit 2-29. Corresponding Design Speeds in Metric and US Customary Units 



A pertinent consideration in selecting design speeds is the average trip length. The longer the 
trip, the greater the driver's desire to use higher speeds. In the design of a substantial length of 
highway, it is desirable to select a uniform design speed. Hov^ever, changes in terrain and other 
physical controls may dictate a change in design speed on certain sections. If so, the introduction 
of a lower design speed should not be done abruptly but should be effected over sufficient 
distance to permit drivers to gradually change speed before reaching the highway section with the 
lower design speed. 

Where it is appropriate to reduce horizontal and vertical alignment features, many drivers 
may not perceive the lower speed condition ahead, and therefore, it is important that they be 
warned well in advance. The changing condition should be indicated by such controls as speed- 
zone and curve-speed signs. 

On rural highways and on high-type urban facilities, a percentage of vehicles is usually able 
to travel at near the free-flow speed governed by geometric design elements; therefore, the 
selection of an appropriate design speed is particularly important. However, in many arterial 
streets, vehicle speeds during several hours of the day are limited or regulated more by the 
presence of large volumes of vehicles and by traffic control devices, rather than by the physical 
characteristics of the street. In such cases, the selection of a design speed is less critical to safe 
and efficient operation. 

During periods of low-to-moderate volume, speeds on arterial streets are governed by such 
factors as posted speed limits, midblock turns into and out of driveways, intersectional turns, 
traffic signal spacing, and signal timing for progression. When arterial street improvements are 
being planned, factors such as future posted speed limits, physical and economic constraints, and 
running speeds likely to be attained during off-peak hours should be considered. All of these 
factors should influence the selection of an appropriate design speed. 



70 



Design Controls and Criteria 



Horizontal alignment generally is not the governing factor in restricting speeds on arterial 
streets. Proposed improvements generally are patterned to the existing street system, and minor 
horizontal alignment changes are commonly made at intersections. The effect of these alignment 
changes is usually small because operation through the intersection is regulated by the type of 
traffic controls needed to handle the volume of cross and turning traffic. Superelevation may be 
provided at curves on urban arterial streets, but the amount of superelevation needed is 
determined in a different manner than for open-road rural conditions. Wide pavement areas, 
proximity of adjacent development, control of cross slope and profile for drainage, and the 
frequency of cross streets and entrances all contribute to the need for lower superelevation rates 
on urban arterial streets. The width of lanes, offset to curbs, proximity of poles and trees to the 
traveled way, presence of pedestrians within the right-of-way, and nearness of business or 
residential buildings, individually or in combination, often limit speeds even on highways with 
good alignment and flat profiles. Despite these factors, designers should strive for good alignment 
and flat profiles in the design of urban arterial streets, since safety and operating characteristics 
can be improved, particularly during off-peak periods. Chapter 3 provides guidance on horizontal 
alignment design for low-speed urban conditions. 

Topography can materially affect the choice of design speed on arterial streets. Many cities 
were developed along watercourses and include areas varying from gently rolling to mountainous 
terrain. Streets may have been constructed originally with only minor grading to fit the 
topography. Because an arterial street is usually developed to fit the alignment of an existing 
street, both through business and residential areas, it generally follows a varying vertical profile. 
Once the design speed is selected, appropriate sight distance should be provided at all crests and 
across the inside of horizontal curves. Profiles with long, continuous grades should be designed 
with proper consideration for the speeds of mass transit and commercial vehicles. Extra lanes on 
the upgrades may be needed so that the grade can match other portions of the facility in capacity 
and enable vehicles that can proceed at a reasonable speed to pass slower moving vehicles. 

Urban arterial streets should be designed and control devices regulated, where practical, to 
permit running speeds of 30 to 75 km/h [20 to 45 mph]. Speeds in the lower portion of this range 
are applicable to local and collector streets through residential areas and to arterial streets through 
more crowded business areas, while the speeds in the higher portion of the range apply to high- 
type arterials in outlying suburban areas. For arterial streets through crowded business areas, 
coordinated signal control through successive intersections is generally needed to permit 
attainment of even the lower speeds. Many cities have substantial lengths of signal controlled 
streets that operate at speeds of 20 to 40 km/h [15 to 25 mph]. 

Under less crowded conditions in suburban areas, it is common on preferred streets to adopt 
some form of speed zoning or control to limit high operating speeds, hi such areas, pedestrians 
along the arterial or vehicles on cross streets, although relatively infrequent, may be exposed to 
potential collisions with through drivers. Such through drivers may gradually gain speed as urban 
restrictions are left behind or may retain their open-road speeds as they enter the city. Thus, 
although through traffic should be expedited to the extent practical, it may be equally important to 
limit speeds to reduce the risk of crashes and to serve local traffic. 



71 



AASHTO — Geometric Design of Highways and Streets 



Posted speed limits, as a matter of policy, are not the highest speeds that might be used by 
drivers. Instead, such limits are usually set to approximate the 85th percentile speed of traffic as 
determined by measuring the speeds of a sizable sample of vehicles. The 85th-percentile speed is 
usually within the "pace" or the 15-km/h [10-mph] speed range used by most drivers. Speed 
zones cannot be made to operate properly if the posted speed limit is detemriined arbitrarily. In 
addition, speed zones should be determined from traffic engineering studies, should be consistent 
with prevailing conditions along the street and with the cross section of the street, and should be 
capable of reasonable enforcement. 

Urban arterial streets and highways generally have running speeds of 30 to 70 km/h [20 to 
45 mph]. It follows that the appropriate design speeds for arterials should range from 50 to 100 
km/h [30 to 60 mph]. The design speed selected for an urban arterial should depend largely on the 
spacing of signalized intersections, the selected type of median cross section, the presence or 
absence of curb and gutter along the outside edges of the traveled way, and the amount and type 
of access to the street. Reconstructed urban arterial highways should generally be designed for an 
operating speed of at least 50 km/h [30 mph]. 

The preceding discussion describes the considerations in selecting an appropriate design 
speed. From this discussion, it should be evident that there are important differences between the 
design criteria applicable to low- and high-speed designs. Because of these distinct differences, 
the upper limit for low-speed design is 70 km/h [45 mph] and the lower limit for high-speed 
design is 80 km/h [50 mph]. 



Traffic Flow Relationships 

Traffic flow conditions on roadways can be characterized by the volume flow rate expressed 
in vehicles per hour, the average speed in kilometers per hour [miles per hour], and the traffic 
density in vehicles per kilometer [vehicles per mile]. These three variables — ^volume, speed, and 
density — are interrelated and have predictable relationships. The generalized relationships 
between volume, speed, and density for uninterrupted flow facilities, as presented in the HCM 
(15) are shown in Exhibit 2-30. The relationships shown in the exhibit are conceptual in nature 
and do not necessarily correspond to the actual relationships used in specific HCM procedures. 
For example, the HCM procedures for freeways and multilane highways show that speed does not 
vary with volume through most of the low and intermediate volume range, as shown in the 
exhibit. The HCM procedures for two-lane highways show that speed varies linearly with volume 
throughout the entire volume range from zero to capacity. 

Density, the number of vehicles per unit length of roadway, increases as vehicles crowd 
closer together. As Exhibit 2-30 shows, when speeds decrease, increased crowding can occur and 
drivers can comfortably follow more closely behind other vehicles. Density is used in the HCM 
as the measure of quality of traffic service for freeways and multilane highways. 

Traffic volumes also vary with density from zero to maximum flow rate, as shown in 
Exhibit 2-30. The two points of zero flow in the exhibit represent either no vehicles at all or so 



72 



Design Controls and Criteria 



many vehicles on the roadway that flow has stopped. The maximum flow is reached at the point of 
maximum density. 



Sf 


V 


CO 


\. 






\ 




Density (v^mi/ln) 



Flow (veh/h/ln) 



Vra 




\ 
\ 
\ 



legend 
Oversaturated flow 



Density (veh/mi/ln) 



Exhibit 2-30. Generalized Speed-Voliime-Density Relationships (15) 

Interference to traffic flow causes speeds to be reduced, vehicles to travel closer together, and 
density to increase, hiterference may be caused by weather conditions, cross traffic, disabled vehicles, 
crashes, or other conditions. As these conditions cause more interference, the flow rates within certain 
limits can still be maintained but with reduced speed, closer vehicle spacing, and greater density. 
When interference becomes so great (despite closer vehicle spacing and greater density) that the 
average speed drops below that necessary to maintain stable flow, there is a rapid decrease in speed 
and traffic flow, and severe congestion occurs. 

When traffic on a highway encounters interference that limits or reduces the roadway capacity in 
a single area, the result is a "bottleneck.'' If the flow entering this bottleneck does not exceed its 
capacity, flow remains stable and no problems arise. However, when the upstream section carries 
more vehicles than the bottleneck can accommodate, a breakdown in traffic flow results. Speeds are 
reduced to a crawl and vehicles begin to queue upstream until incoming flow again falls below the 
outflow capacity. To avoid bottleneck situations, care should be taken to design roadways with 
consistent volume-carrying capacity. The level-of -service concept discussed in the next section helps 
in obtaining this consistency. 

An intersection is often an unavoidable bottleneck. This reduction in capacity becomes acute 
when the intersection is controlled by stop signs or traffic signals. At a traffic signal, vehicles that 
arrive during the red phase encounter a zero-capacity bottleneck. These vehicles form a queue until 
the green phase begins, removing the restraint, and discharging the queue. If the incoming volume is 
too high, not all vehicles in the queue can be discharged during the green phase, and there is 
a continuing buildup of the queue. 



73 



AASHTO — Geometric Design of Highways and Streets 



Arrivals at the intersection are generally predictable in urban areas where the approaching 
vehicles are platooned by upstream signals. In suburban or rural locations, vehicle arrivals are often 
random. This random arrival pattern should be recognized in the design of appropriate cycle times, 
tum-lane storage lengths, and approach capacity. 

At bottlenecks where the traffic must slow down or stop, each vehicle and its occupants incur a 
certain delay. Delays increase fuel consumption and air pollution, which create undesirable economic 
and environmental effects. 



HIGHWAY CAPACITY 
General Characteristics 

The term "capacity" is used to express the maximum hourly rate at which persons or vehicles 
can reasonably be expected to traverse a point (i.e., a uniform section of a lane or a roadway) during a 
given time period under prevailing roadway and traffic conditions. The range of traffic flow on a 
highway can vary from very light volumes to volumes that equal the capacity of the facility as defined 
above. In the generic sense, the term also encompasses broader relations between highway 
characteristics and conditions, traffic composition and flow patterns, and the relative degree of 
congestion at various traffic volumes. Highway capacity issues in this broad sense are discussed 
below. 

The following sections provide a brief overview of the principles and major factors concerning 
highway design capacity. To determine the capacity for a particular highway design, the designer 
should refer to the most recent edition of the Highway Capacity Manual (HCM) (15) for guidance. 
The HCM is used as the basic reference for the following discussion. 

Application 

Highway capacity analysis serves three general purposes, including: 

® Transportation planmiig studies— Highway capacity analysis is used in these studies to 
assess the adequacy or sufficiency of existing highway networks to service current traffic. 
In addition, it is used to estimate the time in the future when traffic growth may overtake 
the capacity of a highway or perhaps reach a level of congestion below capacity that is 
considered undesirable. 

® Highway design — A knowledge of highway capacity is essential to properly fit a planned 
highway to traffic demands. Highway capacity analysis is used both to select the highway 
type and to determine dimensions such as the number of lanes and the minimum lengths for 
weaving sections. 

® Traffic operational analyses— Highway capacity analysis is used in these analyses for 
many purposes, but especially for identifying bottleneck locations (either existing or 
potential). It is also used in preparing estimates of operational improvements that may 

74 



Design Controls and Criteria 



be expected to result from prospective traffic control measures or from spot alterations 
in the highway geometry. 

The traffic data for these uses varies w^ith the degree of accuracy needed. For traffic- 
operational analyses, in which the success of minor improvements may be measured in terms of a 
few vehicles per hour, a high degree of precision is desirable. For highway design, a much lower 
order of precision suffices because the traffic data are frequently estimated for a period 10 to 20 
years in the future and involve not only approximations of traffic volumes but also 
approximations of such factors as traffic composition and movement patterns. The discussion 
below shows the appropriate level of detail to ensure a reasonable balance between the design of 
the highway and the estimated future traffic. Such an analysis ensures that future operating 
conditions will not fall below an acceptable level. If a greater accuracy than is available from the 
suggested procedures is needed, refer to the HCM and other reports on traffic operational 
analysis. 



Capacity as a Design Control 

Design Service Flow Rate Versus Design Volume 

The design volume is the volume of traffic projected to use a particular facility during the 
design year, which is usually 10 to 20 years in the future. Design volumes are estimated in the 
planning process and are often expressed as the expected traffic volume during a specified design 
hour. The derivation of the DHV has been discussed earlier in this chapter in the section on 
'Traffic Characteristics." 

Design service flow rate is the maximum hourly flow rate of traffic that a highway with 
particular design features would be able to serve without the degree of congestion falling below a 
pre-selected level, as described below. 

A major objective in designing a highway is to create a facility with dimensions and 
alignment that can serve the design service flow rate, which should be at least as great as the flow 
rate during the peak 15 -minute period of the design hour, but not so great as to represent an 
extravagance in the design. Where this objective is accomplished, a well-balanced, economical 
highway facility will result. 

Measures of Congestion 

Three key considerations in geometric design are the roadway design, the traffic using the 
roadway, and the degree of congestion on the roadway. The first two considerations can be 
measured in exact units. For example, the roadway either is or is not a highway with full control 
of access, its cross-section dimensions can be expressed in meters [feet], and the steepnesses of 
its grades can be expressed as a percentage. Likewise, traffic flow can be expressed as the number 
of vehicles per unit of time, traffic composition can be expressed as the percentage of vehicles of 



75 



AASHTO — Geometric Design of Highways and Streets 



each class, and the peaking characteristics and directional distribution of traffic can also be 
quantified. 

A scale of values for expressing the degree of congestion is, however, a much more elusive 
measure. Numerous measures of the overall service provided by a roadway section have been 
suggested, including safety, freedom to maneuver, the ratio of traffic volume to capacity (v/c), 
operating speed, average running speed, and others, hi the case of signalized intersections, the 
stopped delay encountered by motorists is a commonly used measure of congestion. 

For uninterrupted traffic flow (i.e., flow not influenced by signalized intersections), traffic 
operational conditions are defined by using three primary measures: speed, volume (or rate of flow), 
and density. Density describes the proximity of vehicles to one another and reflects the freedom to 
maneuver within the traffic stream. It is a critical parameter describing traffic operations with 
uninterrupted flow. As density increases from zero, the rate of flow also increases because more 
vehicles are on the roadway. While this is happening, speed begins to decline (due to the vehicle 
interactions). This decline is virtually negligible at low densities and flow rates. However, as density 
continues to increase, a point is reached at which speed declines noticeably. A maximum rate of flow 
is eventually reached at which the high density of traffic results in markedly decreased speeds and a 
reduced flow rate. This maximum rate of flow for any given facility is defined as its capacity. As 
capacity is approached, flow becomes more unstable because available gaps in the traffic stream 
become fewer and fewer. At capacity, there are no usable gaps in the traffic stream, and any conflict 
from vehicles entering or leaving the facility, or from internal lane changing maneuvers, creates a 
disturbance that cannot be effectively damped or dissipated. Thus, operation at or near capacity is 
difficult to maintain for long periods of time without the formation of upstream queues, and forced or 
breakdown flow becomes almost unavoidable. For this reason, most facilities are designed to operate 
at volumes less than their capacity. 

For interrupted flow, such as that occurring on streets where traffic is controlled by signals, the 
highway user is not as concerned with attaining a high travel speed as with avoiding lengthy stops at 
intersections or a succession of stops at several intersections. Average stopped-time delay is the 
principal measure of effectiveness used in evaluating signalized intersections. Stopped-time delay, 
which is used because it is reasonably easy to measure and is conceptually simple, is a characteristic 
of intersection operations that is closely related to motorist perceptions of quality of traffic flow. 



Relatiori Between Congestion and Traffic Flow Rate 

Congestion does not necessarily signify a complete stoppage of traffic flow. Rather it can be 
thought of as a restriction or interference to normal free flow. For any given class of highway, 
congestion increases with an increase in flow rate until the flow rate is almost equal to the facility's 
capacity, at which point congestion becomes acute. The gradual increase in congestion with increase 
in flow rate is apparent no matter what measure is used as an index of congestion. 



76 



Design Controls and Criteria 



The relationship between running speed and traffic flow rate for freeways, multilane 
highways, and two-lane highways has been discussed earlier in this chapter in the section on 
"Running Speed." As the traffic flow rate approaches a facility's capacity, as defined in the 
HCM (15), any minor disruption in the free flow of traffic may cause traffic on a roadway to 
operate on a stop-and-go basis, with a resulting decrease in traffic flow rate that can be served. 

Highway sections where the paths of traffic must merge and diverge within relatively short 
distances are called "weaving sections." Average running speed, and hence the degree of 
congestion, is a function not only of the volume of traffic involved in the weaving (crossing) 
movements but also of the distance within which the weaving maneuvers must be completed. 
(Weaving is addressed under a separate subsection later in this chapter.) 

On arterial streets within the urban environment, average running speed varies only slightly 
with changes in traffic flow rate. However, delay at signalized intersections may increase 
dramatically as flow rates approach capacity. Therefore, greater degrees of congestion occur, and 
this results in reduced overall travel speeds, higher average travel times, and traffic spill-backs 
into upstream intersections. 



Acceptable Degrees of Congestion 

From the standpoint of the highway user, it would be preferable for each user to have an 
exclusive right to the highway at the time the motorist finds occasion or need to use it. Moreover, 
a motorist would prefer that all highways be of types that would permit speeds far in excess of 
those normally afforded by urban surface streets. However, users recognize that if others are to 
share in the costs of transportation facilities, they are also entitled to share in their use. Therefore, 
they will readily accept a moderate amount of congestion. Just what degree of congestion the 
motoring public is willing to accept as reasonable remains a matter of conjecture, but it is known 
to vary with a number of factors. 

The average motorist understands in a general sense that corrective measures to alleviate 
congestion may be more costly in some instances than in others. As a result, motorists will 
generally accept a higher degree of congestion in those areas where improvements can be made 
only at a substantial cost. Also, motorists are more willing to accept a higher degree of restraint in 
short trips than they are in long trips, but motorists are generally not satisfied with the type of 
operation that occurs when the volume of traffic approaches the facility's capacity. 

From a highway administrator's point of view, the degree of congestion that highway users 
experience is geared to the availability of resources. Historically, funds have never been sufficient 
to meet all needs, causing severe strain in improving highways rapidly enough to prevent the 
traffic demand from exceeding the capacity of the facility. 

The appropriate degree of congestion that should be used in planning and designing highway 
improvements is determined by weighing the desires of the motorists against the resources 
available for satisfying these desires. The degree of congestion that should not be exceeded 
during the design year on a proposed highway can be realistically assessed by: (1) determining 

77 



AASHTO — Geometric Design of Highways and Streets 



the operating conditions that the majority of motorists will accept as satisfactory, (2) determining 
the most extensive highway improvement that the governmental jurisdiction considers practical, 
and (3) reconciling the demands of the motorist and the general public with the finances available 
to meet those demands. 

This reconciliation of desires with available resources is an administrative process of high 
importance. The decision should first be made as to the degree of congestion that should not be 
exceeded during the design period. The appropriate design for a particular facility (such as 
number of lanes) can then be estimated from the concepts discussed in the following sections. 



Principles for Acceptable Degrees of Congestion 

No scientific method exists for deciding the maximum degree of congestion that might be 
accepted as a basis for design. This decision lends itself neither to a modeling technique nor to the 
insertion of coefficients into a computer program. Nevertheless, some principles or guidelines 
that should aid in arriving at such decisions are itemized and discussed on the following pages. 

1. The highway should be so designed that, when it is carrying the design volume, the 
traffic demand will not exceed the capacity of the facility even during short intervals of 
time. 

Conditions can become intolerable for the motorist when the traffic demand exceeds the 
capacity of the street or highway. Moreover, when stop-and-go traffic develops due to congestion 
on highways (other than those controlled by signals), the flow rate that can be served by the 
highway is drastically reduced. Stoppages will occur if the capacity is exceeded even for short 
intervals of time. Because traffic does not flow uniformly throughout a full hour, allowance 
should be made for peaking within the hour. This allowance is made in the HCM procedures with 
an adjustment known as the "peak hour factor," which is discussed later in this chapter. 

Where traffic is controlled by signals at intersections, the relationship between delay and 
capacity may be extremely complex. It is possible to have unacceptably large delays and long 
queues where traffic demand approaches 75 to 85 percent of capacity. The reverse is also 
possible — an intersection approach where traffic demand equals capacity may have low delays if 
the signal cycle is short and/or if signal progression is possible. 

2. The design volume per lane should not exceed the rate at which traffic can dissipate 
from a standing queue. 

This principle is applicable primarily to freeways and other high-type multilane highways. 
For example, if traffic on a freeway lane is stopped even momentarily, it cannot recover at a rate 
equal to the capacity of a freely flowing lane. If the traffic demand exceeds the rate at which cars 
can depart from the head of a standing queue, the queue will increase in length rather than 
dissipate, even after the cause of the stoppage is removed. The rate at which vehicles can depart 
from a standing queue is estimated by various authorities as being within the range of 1,500 to 
1 ,800 passenger cars per lane per hour. 

78 



Design Controls and Criteria 



3. Drivers should be afforded some choice of speed. The latitude in choice of speed should 
be related to the length of trip. 

This principle is applicable to all types of streets and highways. The degree of freedom that 
should be afforded is a subjective determination. On congested freeways with average speeds of 
about 100 km/h [60 mph], for example, the range of speeds between the slowest and fastest driver 
would typically be about 25 km/h [15 mph]. This may be satisfactory for short trips. 

For longer trips, higher average speeds may be warranted, perhaps 10 km/h [5 mph] higher 
than for short trips in densely developed areas. An average speed of 110 km/h [70 mph] or more 
can be achieved on freeways with low to moderate traffic volumes. However, the high cost of 
construction of urban freeways and the impact on the surrounding neighborhoods usually works 
against achieving an operating speed this high except in suburban areas, as discussed further 
under Principle 6 below. 

4. Operating conditions should be such that they provide a degree of freedom from driver 
tension that is related to or consistent with the length and duration of the trip. 

This principle may appear to be a corollary of the previous principle. However, Principle 3 
represents tensions stemming from impatience, whereas this one deals with tensions that develop 
from driving in a dense traffic stream at speeds that an individual driver may consider to be too 
fast for comfort but over which that driver is powerless to exercise control. If the driver reduces 
speed, this induces others to pass and cut in front of them, thereby reducing the gap that the driver 
was seeking to enlarge. Freeway travel at speeds of 65 to 100 km/h [40 to 60 mph] under very 
high-density conditions is a rather tense experience to many and is one that should not be endured 
if avoidable. Presendy, no research data exist to support any recommendations as to the 
maximum length of time that drivers can or should endure travel under high-density conditions, 
but it is commonly accepted that tensions build up with continued exposure. 

Driver tensions associated with freeway densities of 26 passenger cars per kilometer per lane 
[42 passenger cars per mile per lane] or less are generally considered acceptable for trips within 
most metropolitan areas. For long trips the mental concentration that is required and the tensions 
that develop while driving in such heavy traffic are excessive; consequently, lower volumes 
should be used for designing freeways that serve relatively long trips. 

5. There are practical limitations that preclude the design of an ideal freeway. 

An ideal section of freeway would have wide lanes and shoulders on tangent alignment with 
no restrictions in lateral clearance. Such a freeway would be capable of carrying the capacity 
specified for basic freeway segments in the HCM (15). More often than not, it is necessary to 
compromise design features to fit the freeway (or other arterial) within attainable right-of-way, to 
economize on certain features such as curvature or lengths of speed-change lanes, or to locate 
interchanges closer to each other than would be desirable. It is usually not practical to design a 
section of freeway with uniform capacity throughout its length. 



79 



AASHTO — Geometric Design of Highways and Streets 



6. The attitude of motorists toward adverse operating conditions is influenced by their 
awareness of the construction and right-of-way costs that might be necessary to provide 
better service. 

Highway users will accept poor operating conditions if they perceive that the highway is the 
best design that can be reasonably provided at the particular location. They recognize in a general 
way that highways are extremely costly in densely developed areas with high land values, in 
difficult terrain, and at major obstacles to be crossed, such as navigable streams or harbors. 
Consequently, they will accept poorer operating conditions where highway costs are high than 
where there is no apparent reason for deficiencies that can be corrected at moderate expense. 
Because construction costs are frequently much higher in large cities than in small cities, the net 
result is that this principle tends to offset Principle 3 insofar as the effect of trip length within 
densely developed areas is concerned. 



Reconciliation of Principles for Acceptable Degraos of Congestion 

As noted above, the capacities for the base conditions presented in the HCM (15) may not 
be obtainable or desirable on specific highway facilities, depending on the design and desired use 
of the highway. These principles point to the broad general conclusions that are summarized 
below. 

Freeways, For short trips, tolerance to congestion is governed to a considerable extent by 
driving tensions. Loss of travel time is of secondary importance, except that complete stoppages, 
or stop-and-go driving, may be intolerable. These considerations suggest that the density of traffic 
on urban freeways preferably should not exceed 26 passenger cars per kilometer per lane 
[42 passenger cars per mile per lane]. Furthermore, if density does not exceed this level, little 
difficulty from momentary stoppages will result, and minor design restrictions will have no 
noticeable adverse effect on operadng conditions. 

For longer trips in metropolitan areas, travel time becomes more important to the user. 
Driver tensions associated with densities of 26 passenger cars per kilometer per lane [42 
passenger cars per mile per lane], while not unbearable, are decidedly unpleasant. No criteria are 
available for fixing upon any definite value, but indications point to 20 passenger cars per 
kilometer per lane [30 passenger cars per mile per lane] as resulting in an acceptable degree of 
congestion. 

For rural freeways, travel speed is the dominant consideration. On the basis of past 
experience, a density of 13 passenger cars per kilometer per lane [20 passenger cars per mile per 
lane] will permit desirable operations in rural areas. 

Other Miiltilane Highways, Except where traffic is controlled by signals, measures of 
congestion on other multilane highways are similar to those for freeways. Where the interference 
with traffic from marginal development is slight, the traffic densities that result in acceptable 
degrees of congestion on freeways may also be served by other multilane highways. This 
situation is notably true in rural areas, hi urban areas, the traffic volumes that can be served on 

80 



Design Controls and Criteria 



other multilane highways, at acceptable levels of congestion, are generally somewhat lower than 
those for freeways, as will be discussed subsequently in this chapter. 



Factors Other Than Traffic Volume That 
Affect Operating Conditions 

The abiUty of a highway to serve traffic efficiently and effectively is influenced by the 
characteristics of the traffic and by the design features of the highway. 



Highway Factors 

Few highways have ideal designs. Although most modem freeways have adequate cross- 
sectional dimensions, many are not ideal with respect to design speed, weaving section design, 
and ramp terminal design. Inadequacies in these features will result in inefficient use of the 
remaining portions of the freeway. 

On other classes of multilane highways, intersections, even though unsignalized, often 
interfere with the free-flow operation of traffic. Development adjacent to the highway with 
attendant driveways and interference from traffic entering and leaving the through-traffic lanes 
cause a loss in efficiency and lead to congestion and safety problems at relatively low volumes. 
The adverse effect, although readily apparent, can be difficult to quantify (16). Sharp curves and 
steep grades cannot always be avoided, and it is sometimes appropriate to compromise on cross- 
sectional dimensions. All of these conditions combine to cause the effects of congestion to be felt 
at lower traffic volumes than would be the case for highways designed with ideal features and 
protected by full access control or by access management. 

For urban streets with signalized intersections at relatively close intervals, the traffic 
volumes that could otherwise be served are reduced because a portion of each signal cycle must 
be assigned exclusively to the crossing highway. 

For a highway that is deficient in some of its characteristics and where the traffic stream is 
composed of a mixture of vehicle classes rather than passenger cars only, compensatory 
adjustment factors need to be applied to the traffic flow rates used as design values for ideal 
highway conditions. These adjustments are necessary to determine the volume of mixed traffic 
that can be served under minimum acceptable operating conditions on the highway under 
consideration. 

The HCM (15) identifies significant highway features that may have an adverse effect on 
operating conditions. The HCM provides factors and outlines procedures for determining the 
traffic volumes that can be served by highways that are not ideal in all respects. Features that 
could result in a highway being less than ideal in its operational characteristics include narrow 
lanes and shoulders, steep grades, low design speed, and the presence of intersections, ramp 
terminals, and weaving sections. The HCM should be referred to for a discussion of these features 



81 



AASHTO — Geometric Design of Highways and Streets 



and their effects on operating conditions. However, the HCM discussion concerning horizontal 
aUgnment, weaving sections, and ramp terminals is supplemented and amplified below. 



Alignment 

For traffic traveling at any given speed, the better the roadway alignment, the more traffic it 
can carry. It follows that congestion will generally be perceived at lower volumes if the design 
speed is low than if the design speed is high. The highway should be subdivided into sections of 
consistent geometric design characteristics for analysis using the HCM techniques. A single 
limiting curve or steep grade in an otherwise gentle alignment will thus be identified as the 
critical feature linniting roadway capacity. 



Weaving Sections 

Weaving sections are highway segments where the pattern of traffic entering and leaving at 
contiguous points of access results in vehicle paths crossing each other. Where the distance in 
which the crossing is accomplished is relatively short in relation to the volume of weaving traffic, 
operations within the highway section will be congested. Some reduction in operating efficiency 
through weaving sections can be tolerated by highway users if the reduction is minor and the 
frequency of occurrence is not high. It is generally accepted that a reduction in operating speed of 
about 10 km/h [5 mph] below that for which the highway as a whole operates can be considered a 
tolerable degree of congestion for weaving sections. 

Operating conditions within weaving sections are affected by both the length and width of 
the section as well as by the volume of traffic in the several movements. These relationships are 
discussed later in this chapter and in the HCM. 



Ramp Terminals 

Ramps and ramp terminals are features that can adversely influence operating conditions on 
freeways if the demand for their use is excessive or if their design is deficient. When congestion 
develops at freeway ramp junctions, some through vehicles avoid the outside lane of the freeway, 
thereby adding to the congestion in the remaining lanes. Thus, if there are only two lanes in one 
direction, the efficiency per lane is not as high on the average as that for three or more lanes in 
one direction. 

The loss in efficiency is a function of the volume of traffic entering or leaving ramps, the 
distance between points of entry and exit, and the geometric layout of the terminals. Too little is 
known of these separate variables to permit a quantitative assessment of their effect when taken 
individually. Their combined effect is accounted for by levying a uniform assessment against the 
outside lane, regardless of the causes or extent of interference at individual locations. 

Apart from the effect on through traffic, traffic that uses ramps is exposed to a different form 
of congestion, which does not lend itself to measurement in terms of travel speed, delay, or driver 

82 



Design Controls and Criteria 



tension. The degree of congestion for a ramp is related to the total volume of traffic in the outside 
lane of the freeway in the vicinity of the ramp junction (i.e., the combined volume of through 
traffic using the outside lane and the volume of traffic using the ramp). 

The HCM provides procedures for estimating volumes of through traffic in the outside lane 
of a freeway just upstream of an entrance or an exit ramp for various combinations of highway 
and traffic conditions. 



Traffic Factors 

Traffic streams are usually composed of a mixture of vehicles: passenger cars, trucks, buses, 
and, occasionally, recreational vehicles and bicycles. Furthermore, traffic does not flow at a 
uniform rate throughout the hour, day, season, or year. Consideration should be given to these 
two variables, composition of traffic and fluctuations in flow, in deciding upon volumes of traffic 
that will result in acceptable degrees of congestion (see the subsequent discussion on "Levels of 
Service") and also upon the period of time over which the flow should extend. 

The effect of trucks and buses on highway congestion is discussed in the HCM (15). 
Detailed procedures are provided for converting volumes of mixed traffic to equivalent volumes 
of passenger cars. These passenger-car equivalency (PCE) factors used in the HCM differ 
substantially between facility types. 



Peak Hour Factor 

The accepted unit of time for expressing flow rate is a 1-hour period. It is customary to 
design highways with a sufficient number of lanes and with other features that will enable the 
highway to accommodate the forecasted DHV for the design year, which is frequently 20 years 
from the date of construction. 

Because flow is not uniform throughout an hour, there are certain periods within an hour 
during which congestion is worse than at other times. The HCM considers operating conditions 
prevailing during the most congested 15-minute period of the hour to establish the service level 
for the hour as a whole. Accordingly, the total hourly volume that can be served without 
exceeding a specified degree of congestion is equal to or less than four times the maximum 15- 
minute count. 

The factor used to convert the rate of flow during the highest 15-minute period to the total 
hourly volume is the peak hour factor (PEF). The PHF may be described as the ratio of the total 
hourly volume to the number of vehicles during the highest 15-minute period multiplied by 4. 
The PHF is never greater than 1.00 and is normally within the range of 0.75 to 0.95. Thus, for 
example, if the maximum flow rate that can be served by a certain freeway without excessive 
congestion is 4,200 vehicles per hour during the peak 15-minute period, and further, if the PHF is 
0.80, the total hourly volume that can be accommodated at that service level is 3,360 vehicles, or 
80 percent of the traffic flow rate, during the most congested 15-minute period. 



83 



AASHTO — Geometric Design of Highways and Streets 



Levels of Service 

Techniques and procedures for adjusting operational and highway factors to compensate for 
conditions that are other than ideal are found in the HCM (15). It is desirable that the results of 
these procedures be made adaptable to highway design. 

The HCM defines the quality of traffic service provided by specific highway facilities under 
specific traffic demands by means of a level of service. The level of service characterizes the 
operating conditions on the facility in terms of traffic performance measures related to speed and 
travel time, freedom to maneuver, traffic interruptions, and comfort and convenience. The levels 
of service range from level-of-service A (least congested) to level-of-service F (most congested). 
Exhibit 2-31 shows the general definitions of these levels of service. The specific definitions of 
level of service differ by facility type. The HCM presents a more thorough discussion of the 
level-of-service concept. 



Level of service 


General operating conditions 


A 


Free flow 


B 


Reasonably free flow 


C 


Stable flow 


D 


Approaching unstable flow 


E 


Unstable flow 


F 


Forced or breakdown flow 



Note: Specific definitions of levels-of-service A through F vary by facility 
type and are presented in the HCM (15). 

Exhibit 2-31, General Defiiiitioiis of Levels of Service 



The division points between levels-of-service A through F were determined subjectively. 
Furthermore, the HCM contains no recommendations for the apphcability of the levels of service 
in highway design. Choice of an appropriate level of service for design is properly left to the 
highway designer. The guidance in the preceding discussion should enable the designer to link 
the appropriate degrees of congestion to specific levels of service. The relationship between 
highway type and location and the level of service appropriate for design is sunamarized in 
Exhibit 2-32. This relationship is derived from the criteria for acceptable degrees of congestion, 
as outlined earlier in this discussion. 

As may be fitting to the conditions, highway agencies should strive to provide the highest 
level of service practical. For example, in heavily developed sections of metropolitan areas, 
conditions may make the use of level-of-service D appropriate for freeways and arterials; 
however, this level should be used sparingly and at least level-of-service C should be sought. 



84 



Design Controls and Criteria 





Appropriate 


level of service 


for specified combinations of 


Functional 




area 


and terrain type 










Rural 


Urban and 


class 


Rural level 


Rural roll 


ng 


mountainous 


suburban 


Freeway 


B 


B 




C 


C 


Arterial 


B 


B 




C 


C 


Collector 


C 


C 




D 


D 


Local 


D 


D 




D 


D 



Exhibit 2-32, Guidelines for Selection of Design Levels of Service 



Design Service Flow Rates 

The traffic flow rates that can be served at each level of service are termed "service flow 
rates." Once a particular level of service has been identified as applicable for design, the 
corresponding service flow rate logically becomes the design service flow rate, implying that if 
the traffic flow rate using the facility exceeds that value, operating conditions will fall below the 
level of service for which the facility was designed. 

Once a level of service has been selected, it is desirable that all elements of the roadway are 
designed consistent to this level. This consistency of design service flow rate results in near- 
constant freedom of traffic movement and operating speed, and flow interruptions due to 
bottlenecks can be avoided. 

The HCM supplies the analytical base for design calculations and decisions, but the designer 
should use his or her judgment to select the appropriate level of service. Exhibit 2-32 provides 
guidance that may be used by designers in selecting an appropriate level of service. For certain 
recreational routes or for environmental or land use planning reasons, the designer may possibly 
select a design service flow rate less than the anticipated demand. 

Whether designing an intersection, interchange, arterial, or freeway, the selection of the 
desired level of service should be carefully weighed because the traffic operational adequacy of 
the roadway is dependent on this choice. 

Weaving Sections 

Weaving sections occur where one-way traffic streams cross by merging and diverging 
maneuvers. The principal types of weaving sections are illustrated in Exhibit 2-33. Weaving 
sections are designed, checked, and adjusted so that the level of service is consistent with the 
remaining highway. The design level of service of a weaving section is dependent on its length, 
number of lanes, acceptable degree of congestion, and relative volumes of individual movements. 
Large-volume weaving movements usually result in considerable friction and reduction in speed 
of all traffic. Further, there is a definite limit to the amount of traffic that can be handled on a 
given weaving section without undue congestion. This limiting volume is a function of the 



85 



AASHTO — Geometric Design of Highways and Streets 



distribution of traffic between the weaving movements, the length of weaving section, and the 
number of lanes. 

Weaving sections may be considered as simple or multiple. Exhibit 2-34A shows a simple 
weaving section in which a single entrance is followed by a single exit. A multiple-weaving 
section consists of two or more overlapping weaving sections. A multiple weave may also be 
defined as that portion of a one-way roadway that has two consecutive entrances followed closely 
by one or more exits, or one entrance followed closely by two or more exits, as shown in 
Exhibit 2-34B. Multiple weaving sections occur frequently in urban areas where there is need for 
collection and distribution of high concentrations of traffic. For further information conceming 
the operation and analysis of simple and multiple weaving sections, refer to the HCM. 

The weaving section should have a length and number of lanes based on the appropriate 
level of service, as given in Exhibit 2-32. The HCM presents an equation for predicting the 
average running speed of weaving and non-weaving traffic based on roadway and traffic 
conditions. Level -of-service criteria for weaving sections are based on these average running 
speeds. 



Multilaoe Highways Without Access Control 

Multilane highways may be treated as similar to freeways if major crossroads are infrequent, 
or if many of the crossroads are grade separated, and if adjacent development is sparse so as to 
generate little interference. Even on those highways where such interference is currently only 
marginal, the designer should consider the possibility that by the design year the interference may 
be extensive unless access to the highway is well managed. In most cases, the designer should 
assume that extensive crossroad and business improvements are likely over the design life of the 
facility. 

Where there are major crossroads or where adjacent development results in more than slight 
interference, the facility should be treated as a multilane highway without access control. 



Arterial Streets and Urban Highways 

It is often difficult to establish design service flow rates for arterial streets and urban 
highways because the level of service provided by such facilities does not remain stable with the 
passage of time and tends to deteriorate in an unpredictable manner. However, if the principles of 
access management are applied initially to the street or highway, a high level of operations can be 
maintained over time (16, 17, 18). The capacity of an arterial is generally dominated by the 
capacity of its individual signalized intersections. The level of service for a section of an arterial 
is defined by the average overall travel speed for the section. 



86 



Design Controls and Criteria 









""^-ti 



Exhibit 2»33c Weaviiig Sections 



• Weaving ' 



Simple Weaving 





MuHlpte Weaving 
-8- 

Exhibit 2-34, Simple and Multiple Weaving Sections 



87 



AASHTO — Geometric Design of Highways and Streets 



Intersections 

Design capacities of intersections are affected by a very large number of variables. To the 
extent that these variables can be predicted for the design year, design capacities can be estimated 
by procedures for signalized and unsignalized intersections given in the HCM. The design and 
spacing of signalized intersections should also be coordinated with traffic signal design and 
phasing. 



Pedestrians and Bicycles 

The level of service for pedestrian and bicycle facilities can be evaluated using procedures 
presented in the HCM. 



ACCESS CONTROL AND ACCESS MANAGEMENT 
General Conditions 

Regulating access is called "access control." It is achieved through the regulation of public 
access rights to and from properties abutting the highway facilities. These regulations generally 
are categorized as full control of access, partial control of access, access management, and 
driveway /entrance regulations. The principal advantages of controlling access are the 
preservation or improvement of service and safety. 

The functional advantage of providing access control on a street or highway is the 
management of the interference with through traffic. This interference is created by vehicles or 
pedestrians entering, leaving, and crossing the highway. Where access to a highway is managed, 
entrances and exits are located at points best suited to fit traffic and land-use needs and are 
designed to enable vehicles to enter and leave safely with minimum interference from through 
traffic. Vehicles are prevented from entering or leaving elsewhere so that, regardless of the type 
and intensity of development of the roadside areas, a high quality of service is preserved and 
crash potential is lessened. Conversely, on streets or highways where there is no access 
management and roadside businesses are allowed to develop haphazardly, interference from the 
roadside can become a major factor in reducing the capacity, increasing the crash potential, and 
eroding the mobility function of the facility. 

Access control techniques can be implemented with two basic legal powers: police power 
and eminent domain. This first power allows a state to restrict individual actions for the public 
welfare. Police power provides sufficient authority for most access control techniques associated 
with highway operations, driveway location, driveway design, and access denials. The second 
power allows a state to take property for public use provided an owner is compensated for his 
loss. A State may need to use eminent domain when building local service roads, buying abutting 
property, acquiring additional right-of-way, and taking access rights. However, an agency usually 
has the power to deny direct access through the use of police power when reasonable alternative 
access is available. 



Design Controls and Criteria 



Generally, States have adequate power to manage access to a highway as long as reasonable 
access is provided to abutting property. However, providing reasonable access does not 
necessarily mean providing direct access to the state highway system. Coordinating access 
policies into a clear and definitive regulation facilitates the use of police power. Because 
authority and interpretations vary from state to state, each State should evaluate its particular 
legal powers for controlling access. Certain techniques may not be legally feasible in a state that 
has neither the policy nor precedent to uphold them. 

Full control of access means that preference is given to through traffic by providing access 
connections by means of ramps with only selected public roads and by prohibiting crossings at 
grade and direct private driveway connections. 

With partial control of access, preference is given to through traffic to a degree. Access 
connections, which may be at-grade or grade-separated, are provided with selected public roads, 
and private driveways. Generally, full or partial access control is accomplished by legally 
obtaining the access rights from the abutting property owners (usually at the time of purchase of 
the right-of-way) or by the use of frontage roads. 

Access management involves providing (or managing) access to land development while 
simultaneously preserving the flow of traffic on the surrounding road system in terms of safety, 
capacity, and speed (17). Access management applies to all types of roads and streets. It calls for 
setting access pohcies for various types of roadway, keying designs to these policies, having the 
access policies incorporated into legislation, and having the legislation upheld in the courts. 

Access management views the highway and its surrounding activities as part of a single 
system. Individual parts of the system include the activity center and its circulation systems, 
access to and from the center, the availability of public transportation, and the roads serving the 
center. All parts are important and interact with each other. The goal is to coordinate the planning 
and design of each activity center to preserve the capacity of the overall system and to allow 
efficient access to and from the activities. 

Access management extends traffic engineering principles to the location, design, and 
operation of access roads that serve activities along streets and highways. It also includes 
evaluating the suitability of a site for different types of development from an access standpoint 
and is, in a sense, a new element of roadway design. 

Driveway/entrance regulations may be applied even though no control of access is obtained. 
Each abutting property is permitted access to the street or highway; however, the location, 
number, and geometric design of the access points are governed by the regulations. 

Access management addresses the basic questions of when, where, and how access should 
be provided or denied, and what legal or institutional changes are needed to enforce these 
decisions. In a broad context, access management is resource management, since it is a way to 
anticipate and prevent congestion and to improve traffic flow. 



89 



AASHTO — Geometric Design of Highways and Streets 



Key elements of access management include: defining the allowable access and access 
spacings for various classes of highways, providing a mechanism for granting variances when 
reasonable access cannot otherwise be provided, and estabUshing means of enforcing policies and 
decisions. These key elements, along with appropriate design policies, should be implemented 
through a legal code that provides a systematic and supportable basis for making access 
decisions. The code should provide a conmion basis for decisions for both the public and private 
sectors. 



Basic Principles of Access Management 

The following principles define access management techniques: 

® Classify the road system by the primary function of each roadway. Freeways emphasize 

movement and provide complete control of access. Local streets emphasize property 

access rather than traffic movement. Arterial and collector roads must serve a 

combination of both property access and traffic movement. 
® Limit direct access to roads with higher functional classifications. Direct property 

access should be denied or limited along higher class roadways, whenever reasonable 

access can be provided to a lower class roadway. 
® Locate traffic signals to emphasize through traffic movements. Signalized access points 

should fit into the overall signal coordination plan for traffic progression. 
® Locate driveways and major entrances to minimize interference with traffic operations. 

Driveways and entrances should be located away from other intersections to minimize 

crashes, to reduce traffic interference, and to provide for adequate storage lengths for 

vehicles turning into entrances. 
® Use curbed medians and locate median openings to manage access movements and 

minimize conflicts. 

The extent of access management depends upon the location, type and density of 
development, and the nature of the highway system. Access management actions involve both the 
planning and design of new roads and the retrofitting of existing roads and driveways. 



Access Classifications 

Access classification is the foundation of a comprehensive access management program. It 
defines when, where, and how access can be provided between public highways and private 
driveways or entrances. Access classification relates the allowable access to each type of highway 
in conjunction with its purpose, importance, and functional characteristics. 

The functional classification system provides the starting point in assigning highways to 
different access categories. Modifying factors include existing land development, driveway 
density, and geometric design features, such as the presence or absence of a raised-curb median. 



90 



Design Controls and Criteria 



An access classification system defines the type and spacing of allowable access for each 
class of road. Direct access may be denied, limited to right turns in and out, or allowed for all or 
most movements depending upon the specific class and type of road. Spacing of signals in terms 
of distance between signals or through band width (progression speed) is also specified. 
Examples of access classification schemes are presented in NCHRP Report 348, Access 
Management Guidelines for Activity Centers (17). 



Methods of Controlling Access 

Public agencies can manage and control access by means of statutes, land-use ordinances, 
geometric design pohcies, and driveway regulations. 

® Control by the transportation agency: Every State and local transportation agency has 
the basic statutory authority to control all aspects of highway design to protect public 
safety, health, and welfare. The extent to which an agency can apply specific policies 
for driveways/entrances, traffic signal locations, land use controls, and denial of direct 
access is specifically addressed by legislation and, to some degree, by the State courts. 

® Land-use ordinances: Land-use control is normally administered by local 
governments. Local zoning ordinances and subdivision requirements can specify site 
design, setback distances, type of access, parking restrictions, and other elements that 
influence the type, volume, and location of generated traffic. 

® Geometric design: Geometric design features, such as the use of raised-curb medians, 
the spacing of median openings, use of frontage roads, closure of median openings, and 
raised-curb channelization at intersections, all assist in controlling access. 

® Driveway regulations: Agencies may develop detailed access and driveway/entrance 
policies by guidelines, regulations, or ordinances, provided specific statutory authority 
exists. Guidelines usually need no specific authority, but are weak legally. Cities can 
pass ordinances implementing access management policies. Likewise, state agencies 
may develop regulations when authorized by legislation. Regulations can deny direct 
access to a road if reasonable, alternative access is provided, but they cannot "take 
away" access rights. 



Benefits of Controlling Access 

Highways with full access control consistently experience only 25 to 50% of the crash rates 
observed on roadways without access control. These rates are defined in terms of crashes per 
million vehicle kilometers [miles] of travel. Freeways limit the number and variety of events to 
which drivers must respond and thus lower crash rates resuh. 

The safety and operating benefits of controlling access to a highway have long been 
recognized and well documented. As access density increases, there is a corresponding increase 
in crashes and travel times. 



91 



AASHTO — Geometric Design of Highways and Streets 



A Study on congestion by the Texas Transportation Institute has reported a 5- to 8-km/h 
reduction in speed for every added signal per kilometer [2- to 3-mph speed reduction for every 
added signal per rrdlel (19). A research study on the impact of access management found that 
through vehicles in the curb or right lane approximated 20% of the right turns desiring to enter a 
development (18). 

As the number of driveways along a highway increases, the crash rate also increases. The 
effect of driveway and business frequency on crash rates is shown in Exhibit 2-35 through 2-37. 
As the number of business and access points increases along a roadway, there is a corresponding 
increase in crash rates. This contrasts sharply with freeway crash rates that remain the same or 
even decrease slightly over time. 

The generalized effects of access spacing on traffic crashes were derived from a literature 
synthesis and an analysis of 37,500 crashes (18). This study's analysis shows the relative increase 
in crash rates that can be expected as the total driveway density increases. Increasing the access 
frequency from 10 to 30 access points per kilometer [20 to 50 access points per mile] will result 
in almost a doubling of the crash rate. Each additional access point per kilometer increases the 
crash rate about 5 percent; thus, each additional access point per mile increases the crash rate 
about 3 percent. 

Exhibits 2-35 and 2-36 show crash rates by access frequency and type of median for 
urban/suburban and rural roads, respectively. Crash rates rise for each type of median treatment 
with an increase in access frequency. Non-traversable medians generally have a lower crash rate 
than two-way left-turn lanes and undivided roadway sections for all access densities. However, as 
discussed in Chapter 7, provision of non-traversable medians will eliminate left-turn movements 
at some intersections and driveways, but may increase U-turn volumes at other locations on the 
same road or may divert some traffic to other roads. The safety consequences of increased U-turn 
volumes or diverted traffic may not be reflected in Exhibits 2-35 and 2-36. 

For urban/suburban roads, representative crash rates for combinations of signalized and 
unsignalized access density are shown in Exhibit 2-37. This figure indicates that crash rates rise 
with increases in either unsignalized or signalized access density. 

In summary, some degree of access control or access management should be included in the 
development of any street or highway, particularly on a new facility where the likelihood of 
commercial development exists. The type of street or highway to be built should be coordinated 
with the local land-use plan to ensure that the desired type of access can be maintained through 
local zoning ordinances or subdivision regulations. The control of access may range from 
minimal driveway regulations to full control of access. Thus, the extent and degree of access 
management that is practical is a significant factor in defining the type of street or highway. 



92 



Design Controls and Criteria 



1 

it 
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Exhibit 2-35« Estimated Crash Rates by Type of Median — Urban and Suburban Areas (18) 



93 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



-2 1.5 



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0.5 



















































































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US CySTOMARY 









































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Exhibit 2-36. Estimated Crash Rates by Type of Mediae— Rural Areas (18) 



94 



Design Controls and Criteria 



METRIC 



i.75 Signals pcf km ~\ 



. 2J5-3.75 Signals f>cr km [ 



13-2.5 Signals per kra 



^^^ [ < 125 Signals pcf km ] 



20 25 30 

l5j»si^«a1«'««d access poiRi.*! jH'T fern 











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Exhibit 2-37, Estimated Crash Rates by Unsigealized and Signalized Access Density — 

Urban and Suburban Areas (18) 



95 



AASHTO — Geometric Design of Highways and Streets 



THE PEDESTRIAN 
General Considerations 

Interactions of pedestrians with traffic are a major consideration in highway planning and 
design. Pedestrians are a part of every roadway environment, and attention should be paid to their 
presence in rural as well as urban areas. The urban pedestrian, being far more prevalent, more 
often influences roadway design features than the rural pedestrian does. Because of the demands 
of vehicular traffic in congested urban areas, it is often very difficult to make adequate provisions 
for pedestrians. Yet provisions should be made, because pedestrians are the lifeblood of our urban 
areas, especially in the downtown and other retail areas. In general, the most successful shopping 
sections are those that provide the most comfort and pleasure for pedestrians. Pedestrian facilities 
include sidewalks, crosswalks, traffic control features, and curb cuts (depressed curbs and ramped 
sidewalks) and ramps for the older walkers and persons with mobility impairments. Pedestrian 
facilities also include bus stops or other loading areas, sidewalks on grade separations, and the 
stairs, escalators, or elevators related to these facilities. The Americans with Disabilities Act 
Accessibility Guidelines (ADAAG) (23) must be considered when designing roadways where 
pedestrian traffic is expected. 



General Characteristics 

To effectively plan and design pedestrian facilities, it is necessary to understand the typical 
pedestrian. The pedestrian most likely will not walk over 1.5 km [1 mi] to work or over 1.0 km 
[0.5 mi] to catch a bus, and about 80 percent of the distances traveled by the pedestrian will be 
less than 1.0 km [0.5 mi] (24). The typical pedestrian is a shopper about 50 percent of the time 
that he or she is a pedestrian and a commuter only about 11 percent of the time. As a 
consequence, pedestrian volumes peak at about noon rather than at the peak commuter times. 
Pedestrian volumes are influenced by such transient conditions as weather or, in specific 
locations, advertised sales. Hourly fluctuations in pedestrian volumes on a city street can be found 
in the AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities (25). 

Pedestrian actions are less predictable than those of motorists. Many pedestrians consider 
themselves outside the law in traffic matters, and in many cases, pedestrian regulations are not 
fully enforced. This makes it difficult to design a facility for safe and orderly pedestrian 
movements. 

Pedestrians tend to walk in a path representing the shortest distance between two points. 
Therefore, crossings in addition to those at comers and signalized intersections may be 
appropriate at particular locations. 

Pedestrians also have a basic resistance to changes in grade or elevation when crossing 
roadways and tend to avoid using special underpass or overpass pedestrian facilities. Also, 
pedestrian underpasses may be potential crime areas, lessening their usage. 



96 



Design Controls and Criteria 



A pedestrian's age is an important factor that may explain behavior that leads to collisions 
between motor vehicles and pedestrians. Very young pedestrians are often careless in traffic from 
either ignorance or exuberance, whereas older pedestrians may be affected by limitations in 
sensory, perceptual, cognitive, or motor skills. Pedestrian collisions can also be related to the lack 
of sidewalks, which may force pedestrians to share the traveled way with motorists. Therefore, 
sidewalk construction should be considered as part of any urban/suburban street improvement. 

The following have been suggested as measures with the potential to aid older pedestrians 
and road users: 

® Use simple designs that minimize crossing widths and minimize the use of more 
complex elements such as channelization and separate turning lanes. Where these 
features are appropriate, assess alternative designs that will assist older pedestrians, 
such as 3.3-m [11-ft] lane widths. 

® Assume lower walking speeds. 

@ Provide median refuge islands of sufficient width at wide intersections. 

® Provide lighting and eliminate glare sources at locations that demand multiple 
information gathering and processing. 

® Consider the traffic control system in the context of the geometric design to assure 
compatibility and to provide adequate advance warning or guide signs for situations that 
could surprise or adversely affect the safety of older drivers or pedestrians. 

® Use enhanced traffic control devices. 

® Provide oversized, retroreflective signs with suitable legibility. 

® Consider increasing sign letter size and retroreflectivity to accommodate individuals 
with decreased visual acuity. 

® Use properly located signals with large signal indications. 

® Provide enhanced markings and delineation. 

^ Use repetition and redundancy in design and in signing. 

For further information on older pedestrians and drivers, refer to the FHWA publication 
entitled Older Driver Highway Design Handbook: (Recommendations and Guidelines) (6). 



Walking Speeds 

Because pedestrians have a broad range of walking speeds, the speeds at which they may 
cross a street is significant in design. Average pedestrian walking speeds range from 
approximately 0.8 to 1.8 m/s [2.5 to 6.0 ft/sec]. The Manual on Uniform Traffic Control Devices 
(MUTCD) (7) uses a normal walking speed of 1.2 m/s [4.0 ft/s]. Older people will generally walk 
at speeds in the lower end of this range. 

Walking speeds are faster at midblock locations than at intersections, are faster for men than 
for women, and are affected by steep grades. Air temperature, time of day, trip purpose, and ice 
and snow all affect pedestrian walking speeds. Age is the most common cause of slower walking 
speeds, and in areas where there are many older people, a speed of 0.9 m/s [2.8 ft/s] should be 
considered for use in design. 

97 



AASHTO — Geometric Design of Highways and Streets 



Walkway Capacities 

Walking speeds decrease as the pedestrian density of the walkway increases. As with 
roadway capacities, there is an optimum speed and density under which the walkway will carry 
the largest volume. The effective width used for walkway calculations should be reduced where 
parking meters, hydrants, newsstands, litter barrels, utility poles, or similar obstructions preclude 
the use of the full walkway. Walkway capacity calculations for sidewalks, stairs, and the effect of 
traffic signals involve differing procedures as discussed below. For a more detailed analysis of 
walkway design and capacities, see the AASHTO Guide for the Planning, Design, and Operation 
of Pedestrian Facilities (25) and the current edition of the Highway Capacity Manual (15). 



Sidewalks 

Levels of service have been developed to quantify the relative mobility of the pedestrian and 
his or her conflicts with other pedestrians that influence walking speed, maneuvering room, and 
the feeling of comfort (26). As in the level-of-service concept for motor vehicle traffic discussed 
earlier in this chapter and in the HCM (15), levels of service (A to F) reflect increasing crowding 
and decreasing freedom of movement. These levels of service are based on the available area per 
person and are defined as follows (27): 

Level-of-service A allows each person to choose a desired walking speed and to avoid 
conflicts with other pedestrians. 

At Level-of-service B, pedestrians begin to be aware of other pedestrians. 

Level-of-service C requires minor adjustments to speed and direction by pedestrians to avoid 
conflicts. 

At Level-of-service D, freedom to select individual walking speed and bypass other 
pedestrians is restricted. Frequent changes in speed and position are required. 

Level-of-service E provides for very crowded walking, at times reduced to shuffling, making 
reverse or cross-traffic flow very difficult. The speed of virtually all pedestrians is reduced. 

At Level-of-service F, a person is likely to be standing stationary in a waiting area or is able 
to walk only by shuffling. There is frequent, unavoidable contact with other pedestrians. 

Computations of walkway capacity should use walkway widths that are reduced about 
500 mm [18 in] if there are adjacent walls, with an additional 500 mm [18 in] if window shoppers 
are expected. Street hardware such as parking meters and poles also reduces the available 
walkway width. 



98 



Design Controls and Criteria 



Intersections 

When pedestrians encounter an intersection, there is a major interruption in pedestrian flow. 
The sidewalk should provide sufficient storage area for those waiting to cross as well as an area 
for pedestrian cross traffic to pass. 

Once pedestrians are given the walk indication, the crosswalk width and length become 
important. Crosswalks should be wide enough to accommodate the pedestrian flow in both 
directions within the duration of the pedestrian signal phase. The wider the street, the longer it 
takes a pedestrian to cross and proportionately less green signal time will be available for the 
primary street movements. Additionally, the longer the pedestrian crossing time, the longer the 
exposure to potential pedestrian/vehicular conflicts. 

If the intersection is not signal controlled or if stop signs do not control the through motor 
vehicular traffic, pedestrians must wait for suitable gaps in the traffic to cross. The wider the 
street, the longer the gaps must be to afford safe pedestrian crossing times. Under urban 
conditions, pedestrian crossing times may be reduced by using narrower lanes or by providing 
raised-curb medians. However, traffic safety and reasonable roadway and intersection capacity 
requirements should still be met when considering reduced crossing times. 



Reducing Pedestrian-Vehicular Conflicts 

When designing urban highways with substantial pedestrian-vehicular conflicts, the 
following are some measures that could be considered to help reduce these conflicts and may 
increase the efficient operation of the roadway: (1) eliminate left and/or right turns, (2) prohibit 
free-flow right-turn movements, (3) prohibit right tum on red, (4) convert from two-way to one- 
way street operation, (5) provide separate signal phases for pedestrians, (6) eliminate selected 
crosswalks, and (7) provide for pedestrian grade separations. These and other pedestrian 
considerations are detailed in subsequent chapters and in the AASHTO Guide for the Planning, 
Design, and Operation of Pedestrian Facilities (25). 



Characteristics of Persons With Disabilities 

Consideration of persons with disabilities in highway design can greatly enhance the 
mobility of this sector of our society. To adequately provide for persons with disabilities, the 
designer must be aware of the range of disabilities to expect so that the design can appropriately 
accommodate them. The designer is cautioned to adequately review all local and national 
guidelines to assure proper compliance with applicable rules and regulations (26). For further 
details, see the section on "Sidewalk Curb Ramps" in Chapter 4, as well as the AASHTO Guide 
for the Planning, Design, and Operation of Pedestrian Facilities (25) and the ADAAG (23). 



99 



AASHTO — Geometric Design of Highways and Streets 



Mobility Impairments 

Ambulatory difficulties include persons who walk without assistive devices, but with 
difficulty, to persons who require aid from braces, canes, or crutches, to persons who use 
wheelchairs. Stairs, curbs, and raised channeUzing islands are the major roadway obstructions to 
these pedestrians. Design modifications should provide ramps rather than stairs or curbs. The 
front wheels of a wheelchair are very sensitive to obstacles; any bump may impair the progress of 
a wheelchair and may increase the possibility that a user will be propelled out of the wheelchair. 



Visual Impairmerits 

Pedestrians with visual impairments need special consideration. Intersections are the most 
complicated transportation element for visually impaired people. Complicated crossings such as 
those at channelized intersections can be improved by installing guide strips. Sidewalk curb cuts 
for wheelchairs make it difficult for visually impaired pedestrians to locate the curb line. Adding 
a 600-mm [2-ft] detectable warning strip at the bottom of the sidewalk ramp that meets the design 
specifications of the ADAAG (23) will benefit people with visual impairments. Because the 
visually impaired often rely on the sound of traffic when crossing intersections, caution should be 
used when considering exclusive turn phases or other unusual traffic movements. 



Developmental Impairments 

Many people with developmental impairments are unable to drive and, therefore, often travel 
as pedestrians. To help ensure correct responses from these pedestrians, including young children, 
pedestrian signals or other pedestrian-related facilities should be simple, straightforward, and 
consistent in their meaning. 



BICYCLE FACIUTIES 

The bicycle has become an important element for consideration in the highway design 
process. Fortunately, the existing street and highway system provides most of the mileage needed 
for bicycle travel. While many highway agencies allow bicycles on partially access controlled 
facilities, most highway agencies do not allow bicycles on fully access controlled facilities unless 
no other alternative route is available. 

Improvements such as the following, which generally are of low to moderate cost, can 
considerably enhance the safety of a street or highway and provide for bicycle traffic: 

® paved shoulders 

© wider outside traffic lanes (4.2 m [14 ft] minimum), if no shoulders exist 

® bicycle-safe drainage grates 

® adjusting manhole covers to the grade 

® maintaining a smooth, clean riding surface 

100 



Design Controls and Criteria 



At certain locations or in certain corridors, it is appropriate to further supplement the existing 
highway system by providing specifically designated bikeways (for either exclusive or non- 
exclusive bicycle use). To provide adequately for bicycle traffic, the designer should be familiar 
with bicycle dimensions, operating characteristics, and needs. These factors determine acceptable 
turning radii, grades, and sight distance. In many instances, design features of separate bike 
facilities are controlled by the adjoining roadway and by the design of the highway itself. For 
further guidance, refer to the latest edition of the AASHTO Guide for Development of Bicycle 
Facilities (28) and other current research (29). 



SAFETY 

Attention to highway safety has been emphasized by the Congress of the United States as 
well as other national committees concerned with safety. In July 1973, after hearings on highway 
safety, design, and operations were conducted by subcommittees of the House Committee on 
Public Works, the following mandate was published by the Committee: 

Whose responsibility is it to see that maximum safety is incorporated into our motor 
vehicle transportation system? On this, the subcommittee is adamant. It is the 
responsibility of Government and specifically those agencies that, by law, have been 
given that mandate. This responsibility begins with the Congress and flows through 
the Department of Transportation, its Federal Highway Administration, the State 
Highway Departments and safety agencies, and the street and highway units of 
counties, townships, cities, and towns. There is no retreating from this mandate, 
either in letter or in spirit (30). 

This emphasis by Congress on safety has also been evidenced by passage of the Highway 
Safety Act of 1966, and from the Federal Highway Administration (FHWA) by adoption of the 
AASHTO pubHcations, Highway Design and Operational Practices Related to Highway Safety 
(30) and Highway Safety Design and Operations Guide (31). Other safety resources include the 
report entitled Enhancing Highway Safety in an Age of Limited Resources (32), which resulted 
from the TRB-conducted symposium sponsored by AASHTO and others in 1981. 

Crashes seldom result from a single cause — usually several influences affect the situation at 
any given time. These influences can be separated into three groups: the human element, the 
vehicle element, and the highway element. Although this policy is primarily concerned with 
highway characteristics and design, the role of psychological factors is ever present. An error in 
perception or judgment or a faulty action on the driver's part can easily lead to a crash. 

Highways should be designed to minimize driver decisions and to reduce unexpected 
situations. The number of crashes increases with an increase in the number of decisions required 
of the driver. Uniformity in highway design features and traffic control devices plays an 
important role in reducing the number of required decisions, and by this means, the driver 
becomes aware of what to expect on a certain type of highway. 



101 



AASHTO—Geometric Design of Highways and Streets 



The most significant design factor contributing to safety is the provision of full access 
control. Full access control reduces the nunnber, frequency, and variety of events to which drivers 
must respond. The beneficial effect of this element has been documented in reports of a 
cooperative research study (33) of the FHWA and 39 state highway agencies. One of the principal 
findings of this study is that highways without access control invariably had higher crash rates 
than those with access control. This study showed that crash, injury, and fatality rates on 
Interstate highways are between 30 and 76 percent of comparable rates of conventional highways 
that existed before the Interstate highways were opened to traffic. No other single design element 
can claim comparable reductions. 

Research has demonstrated a relationship between crashes and number of access points on a 
roadway (18^ 19<, 34). Relationships of this type have been illustrated in Exhibits 2-35 through 
2-37. 

The principle of full access control is invaluable as a means for preserving the capacity of 
arterial highways and of minimizing crash potential; however, this principle does not have 
universal application. Highways without control of access are essential as land service facilities, 
and the design features and operating characteristics of these highways need to be carefully 
planned so that they will reduce conflicts and minimize the interference between vehicles and still 
meet the needs of highway users. 

Speed is often a contributing factor in crashes, but its role must be related to actual 
conditions at a crash site to be understood. It is improper to conclude that any given speed is safer 
than another for all combinations of the many kinds of drivers, vehicles, highways, and local 
conditions. For a highway with particularly adverse roadway conditions, a relatively low speed 
may result in fewer crashes than a high speed, but this does not necessarily mean that all potential 
crashes can be eliminated by low speeds. Likewise, vehicles traveling on good roads at relatively 
high speed may have lower crash involvement rates than vehicles traveling at lower speeds, but it 
does not necessarily follow that yet a higher speed would be even safer. 

The safest speed for any highway depends on design features, road conditions, traffic 
volumes, weather conditions, roadside development, spacing of intersecting roads, cross-traffic 
volumes, and other factors. Crashes are not related as much to speed as to the range in speeds 
from the highest to the lowest. Regardless of the average speed on a main rural highway, the 
greater a driver's deviation from this average speed, either lower or higher, the greater the 
probability that the driver will be involved in crashes. Thus, design features that reduce the 
variance in speed of vehicles (such as flat grades, speed-change lanes, grade separations, and 
good signing and marking) contribute to highway safety. Normally, crashes involving vehicles 
traveling at high speed are more severe than those at low speed. 

When designing a highway, consideration should be given to the type and characteristics of 
the drivers expected to use the highway. Trip purposes (such as recreation, commuting to work, 
and through travel) are factors affecting the design to some extent. Trip purposes are related to 
the mix of vehicle types likely to use the highway, ranging from all passenger vehicles to a high 
percentage of heavy commercial vehicles. Where trips of one type predominate, the facility 
should be designed to fit the specific needs of that type of trip. 

102 



Design Controls and Criteria 



A study on the effect of the Interstate highway system on crashes found a lower crash rate on 
four-lane divided highways than on four-lane undivided highways (35). This study was developed 
from data for highways within Interstate highway corridors during periods before and after 
opening new sections of Interstate highways to traffic. 

A highway with a median width of 15 m [50 ft] or more has a very low incidence of head-on 
collisions caused by vehicles crossing the median. A median width of 23 to 30 m [75 to 100 ft] on 
freeways is very desirable as a means of reducing cross-median coOisions. On a divided highway 
with partial access control (i.e., an expressway) or where no access control exists, the width of 
median should also take into account the operation of at-grade intersections. 

With narrower medians, median barriers will eliminate head-on collisions, but at the cost of 
some increase in same-direction crashes because recovery space is decreased. Properly designed 
median barriers minimize vehicle damage and lessen the crash severity. However, if a narrow 
median with a median barrier is proposed on a high-speed highway, the design should include 
adequate shoulder widths in the median for emergency stops and emergency vehicle use. 

Another study relating crashes to shoulder width, alignment, and grade found that crash rates 
on sections with curves or grades were much higher than on level tangent highway sections. This 
study also found that crash rates were highest on roads having combinations of sharp curves and 
steep grades (36). This study, which was limited to rural two-lane roads, lends strong support to 
the postulate that straight, level rural roads without intersections or significant numbers of private 
driveways are the safest highways widiin their general class. The few crashes that occur on 
straight, level, rural roads without intersections do not represent a stable source of information, 
however. There are wide variations between similar roadway sections and between different years 
for the same section. An apparent correlation between a geometric design element such as 
shoulder width, for example, and crash rates is almost certain to be clouded by random variations 
of the crash pattern. 

Zegeer (37) developed relationships between the geometric design of horizontal curves and 
their safety performance. This research addresses the relationship to safety of both length and 
radius of horizontal curves. 

Crashes are likely to occur where drivers are called upon to make decisions under 
circumstances where their vehicles are unable to respond properly, for example, where a truck is 
descending a grade. It would be logical to expect more crashes on grades and curves than on level 
tangent highways where driver decisions are needed less frequently and vehicles are fully 
responsive. However, design with tangent alignment can be overdone. 

On extremely long tangents, drivers have a tendency to completely relax, especially after 
driving on a congested highway before entering a freeway. On some freeways, there has been 
concern over the number of crashes that occur when the driver apparently goes to sleep. It is 
considered highly desirable to provide gentle curvature and to avoid a fixed cross section for long 
tangent sections of roadway. This can be achieved by varying the median width, using 
independent roadway alignments, and taking advantage of the terrain, wherever practical. In 



103 



AASHTO — Geometric Design of Highways and Streets 



addition, rumble strips can be added to shoulders to reduce run-off-the-road crashes caused by 
drivers falling asleep at the wheel. 

As the design of alignment, grade, and traveled-way cross section has improved, roadside 
design has also become increasingly important. Crashes involving single vehicles running off the 
road constitute more than one-half of all fatal crashes on freeways. 

When a vehicle leaves the roadway, the driver no longer has the ability to fully control the 
vehicle. Any object in or near the path of the vehicle becomes a potential contributing factor to 
crash severity. The concept of the safer or forgiving roadside should not be viewed as a by- 
product of the application of safety criteria to each element but as a planned segment of the total 
engineering for the highway. The AASHTO publication Highway Safety Design and Operation 
Guide (31) presents an overview of the AASHTO policies in this area; these policies are reflected 
throughout this book in the criteria for specific geometric design elements. 

Basic to the concept of the forgiving roadside is the provision of a clear recovery area. 
Studies have indicated that on high-speed highways, a relatively level traversable width of 
approximately 9 m [30 ft] from the edge of the traveled way permits about 80 percent of the 
vehicles leaving the highway to safely stop or return to the roadway. Even though the 9-m [30-ft] 
width is not a "magic number" and the application of engineering judgment is necessary, the 9-m 
[30-ft] width has been used extensively as a guide for recovery zones. 

In roadside design, two major elements should be controlled by the designer: roadside slopes 
and unyielding obstacles. NCHRP Report 247 (38) discusses the effectiveness of clear recovery 
areas. The AASHTO Roadside Design Guide (39) also discusses the effects that slope and other 
topographic features have on the effectiveness of recovery areas. On existing highways, 
AASHTO recommends the following priorities for treatment of roadside obstacles: 

® Remove the obstacle or redesign it so it can be safely traversed. 

® Relocate the obstacle to a point where it is less likely to be struck. 

® Reduce severity of impacts with the obstacle by using an appropriate breakaway device. 

® Redirect a vehicle by shielding the obstacle with a longitudinal traffic barrier and/or 

crash cushion. 
® Delineate the obstacle if the above alternatives are not appropriate. 

The design of guardrails and barrier systems has become a subject of considerable research. 
AASHTO Roadside Design Guide (39) and NCHRP Report 350 (40) are some of many published 
reports that deal with this subject. These publications note that the treatment of end sections on 
guardrail or a barrier is of particular concern. 

Highway designers should recognize the dynamic developments currently under way in the 
entire area of roadside design. Although this publication has attempted to present the most current 
information available on roadside design, ongoing research and implementation projects will 
undoubtedly offer newer and better results in the future. Highway designers should endeavor to 
use the most current acceptable information in their designs. 



104 



Design Controls and Criteria 



Communication with the motorist is probably one of the most complex problems for the 
designer. One of the best available tools concerning motorist communication is the MUTCD (7), 
which presents national criteria for uniform application of signing, signalization, painted 
channelization, and pavement markings for all highways in the United States. A primary message 
of the MUTCD is the importance of uniformity. 

Highway users are dependent on traffic control devices (signs, markings, and signals) for 
information, warning, and guidance. So great is the dependence of highway users on such 
information that uniform, high-quality traffic control devices are necessary for safe, efficient use 
and public acceptance of any highway regardless of its excellence in width, alignment, and 
structural design. 

All traffic control devices should have the following characteristics: (1) fulfill an important 
need, (2) command attention, (3) convey a clear, simple meaning, (4) command respect of road 
users, and (5) provide adequate response time. In addition, devices that control or regulate traffic 
must be sanctioned by law. 

Four basic attributes of traffic control devices are essential to ensure that these devices are 
effective: design, placement, maintenance, and uniformity. Consideration should be given to 
these attributes during the design of a highway to ensure that the required number of devices can 
be kept to a minimum and that those that are needed can be properly placed. 

The operation of a motor vehicle takes considerable concentration, particularly in congested 
areas. A driver should be able to operate his or her vehicle with minimum distractions. 
Advertising or other roadside signs should not be placed where they would interfere with or 
confuse the meaning of standard traffic control devices. Advertising signs with bright colors or 
flashing lights are especially objecfionable in this respect. Lights shining toward a driver can be 
blinding, partially or fully, for varying periods of time, depending on individual eye capability. 
Bright lights, in effect, can form a curtain hiding what is ahead and can thus put motorists and 
pedestrians at risk. 

A large proportion of crashes on rural highways occur at intersections. Several studies have 
been made at intersections with varying conditions, and the results vary according to conditions 
studied. Factors to be considered in designing an intersection are total traffic volume, amount of 
cross traffic, turning movements, type of highway, type of traffic control needed, design of the 
crossroad sight distance, and the utilization of islands and channelization. 

Various studies indicate improvements in safety at intersections can be accomplished by 
channelizing intersections, providing appropriate sight distances (including stopping, decision, 
and intersection sight distance), and providing safety refuge islands and sidewalks for pedestrians, 
hghting, signing, and traffic control devices. These concepts have been incorporated in the 
geometric design guidelines presented in this policy. 

A viable safety evaluation and improvement program is a vital part of the overall highway 
improvement program. The identification of potential safety problems, the evaluation of the 
effectiveness of alternative solutions, and the programming of available funds for the most 

105 



AASHTO — Geometric Design of Highways and Streets 



effective improvements are of primary importance. The safety of the traveling public should be 
reflected throughout the highway program: in spot safety projects, in rehabilitation projects, in the 
construction of new highways, and elsewhere. Highway Design and Operational Practices 
Related to Highway Safety (30) and Highway Safety Design and Operations Guide (31) provide a 
number of important recommendations on safety as part of a total highway program. 



ENVIRONMENT 

A highway necessarily has wide-ranging effects in addition to providing traffic service to 
users. It is essential that the highway be considered as an element of the total environment. The 
term ''environment'' as used here refers to the totality of humankind's surroundings: social, 
physical, natural, and synthetic. It includes the human, animal, and plant communities and the 
forces that act on all three. The highway can and should be located and designed to complement 
its environment and serve as a catalyst to environmental improvement. 

The area surrounding a proposed highway is an interrelated system of natural, synthetic, and 
sociologic variables. Changes in one variable within this system cannot be made without some 
effect on other variables. The consequences of some of these effects may be negligible, but others 
may have a strong and lasting impact on the environment, including the sustenance and quality of 
human life. Because highway location and design decisions have an effect on the development of 
adjacent areas, it is important that environmental variables be given full consideration. Also, care 
should be exercised to ensure that applicable local, state, and federal environmental requirements 
are met. 



ECONOMIC ANALYSIS 

Highway economics is concerned with the cost of a proposed improvement and the benefits 
resulting from it. The AASHTO Manual on User Benefit Analysis of Highway and Bus-Transit 
Improvements (41) may be used to perform economic analyses of proposed highway 
improvements. 



REFERENCES 

1. Fong, K. T., and D.C. Chenu. ''Simulation of Truck Turns With a Computer Model," 
Transportation Research Record 1100, Transportation Research Board, 1985: 20-29. 

2. Fambro, D. B., K. Fitzpatrick, and R. J. Koppa. Determination of Stopping Sight Distances, 
NCHRP Report 400, Washington, D.C: Transportation Research Board, 1997. 

3. Olson, P. L., D. E. Cleveland, P. S. Fancher, L. P. Kostyniuk, and L. W. Schneider. 
Parameters Affecting Stopping Sight Distance, NCHRP Report 270, Washington, D.C: 
Transportation Research Board, 1984. 

4. Alexander, G. H., and H. Lunenfeld. A User's Guide to Positive Guidance (3rd Edition), 
Report No. FHWA/SA-90/017, Washington, D.C: U.S. Department of Transportation, 
Federal Highway Administration, 1990. 

106 



Design Controls and Criteria 



5. "Human Factors and Safety Research Related to Highway Design and Operations," 
Transportation Research Record 1281, Transportation Research Board, 1990. 

6. Staplin, L., K. Lococo, and S. Byington. Older Driver Highway Design Handbook, Report No. 
FHWA-RD-97-135, McLean, Virginia: U.S. Department of Transportation, Federal Highway 
Administration, December 1998. 

7. U.S. Department of Transportation, Federal Highway Administration. Manual on Uniform Traffic 
Control Devices for Streets and Highways, Washington, D.C.: 1988 or most current edition. 

8. Johannson, C, and K. Rumar. "Driver's Brake Reaction Time," Human Factors, Vol. 13, No. 1, 
1971: 22-27. 

9. Fell, J. C. A Motor Vehicle Accident Causal System. The Human Element, Report No. DOT-HS- 
801-214, Washington, D.C.: U.S. Department of Transportation, National Highway Traffic 
Safety Administration, July 1974. 

10. Schmidt, L, and P. D. Connolly. "Visual Considerations of Man, the Vehicle and the Highways," 
Paper No. SP-279-SAE, New York: Society of Automotive Engineers, 1966. 

11. Tilley, D. H., C. W. Erwin, and D. T. Gianturco. "Drowsiness and Driving; Preliminary Report of 
a Population Survey," Paper No. 730121-SAE, New York: Society of Automotive Engineers, 
1973. 

12. Alexander, G. J., and H. Lunenfeld, Driver Expectancy in Highway Design and Operations, 
Report No. FHWA-TO-86-1, Washington, D.C.: U.S. Department of Transportation, Federal 
Highway Administration, May 1986. 

13. Adler, B., and H. Lunenfeld. Three Beam Headlight Evaluation, Report No. DOT/HS-800-844, 
Washington, D.C.: U.S. Department of Transportation, National Highway Traffic Safety 
Administration, April 1973. 

14. Institute of Traffic Engineers. Freeway Operations, Washington, D.C.: Institute of Traffic 
Engineers, 1961. 

15. Transportation Research Board. Highway Capacity Manual, HCM2000, Washington, D.C.: 
Transportation Research Board, 2000 or most current edition. 

16. Bonneson, J. A., and P. T. McCoy. Capacity and Operational Effects of Midblock Left-Turn 
Lanes, NCHRP Report 395, Washington, D.C.: Transportation Research Board, 1997. 

17. Koepke, F. J. and H. S. Levinson. Access Management Guidelines for Activity Centers, NCHRP 
Report 348, Washington, D.C.: Transportation Research Board, 1992. 

18. Gluck, J., H. S. Levinson, and V. Stover. Impacts of Access Management Techniques, NCHRP 
Report 420, Washington, D.C.: Transportation Research Board, 1999. 

19. Lomax T., S. Turner, H. S. Levinson, R. Pratt, P. Bay, and T. Douglas. Quantifying Congestion, 
NCHRP Report 398, Washington, D.C.: Transportation Research Board, March 1997. 

20. State of Colorado Access Control Demonstration Project. Colorado Department of Highways, 
June 1985. 

21. Cribbins, P. D., J. W. Horn, F. V. Beeson, and R. D. Taylor. "Median Openings on Divided 
Highways: Their Effect on Accident Rates and Level of Service," Highway Research Record 
188, Highway Research Board, 1967. 

22. Glennon, J. C, J. J. Valenta, B. A. Thorson, and J. A. Azzeh. Technical Guidelines for the 
Control of Direct Access to Arterial Highways, Volumes 1 and 2, Report Nos. FHWA-RD-76-87 
and -88, McLean, Virginia: U.S. Department of Transportation, Federal Highway 
Administration, August 1975. 



707 



AASHTO — Geometric Design of Highways and Streets 



23. Architectural and Transportation Barriers Compliance Board (Access Board). Americans With 
Disabilities Act Accessibility Guidelines (ADAAG), Washington, D.C.: July 1994 or most current 
edition. 

24. Maring, G. E. "Pedestrian Travel Characteristics," Highway Research Record 406, Highway 
Research Board 1972: 14-20. 

25. AASHTO. Guide for the Planning, Design, and Operation of Pedestrian Facilities, Washington, 
D.C.: AASHTO, forthcoming. 

26. Older, S. J. "Movement of Pedestrians on Footways in Shopping Streets," Traffic Engineering and 
Control, August 1968: 160-163. 

27. Fruin, J. J. "Designing for Pedestrians: A Level-of-Service Concept," Highway Research Record 355, 
Highway Research Board, 1971: 1-15. 

28. AASHTO. Guide for the Development of Bicycle Facilities, Washington, D.C.: AASHTO, 1999. 

29. Wilkinson, III, W. C, A. Clarke, B. Epperson, and R. L. Knoblauch. Selecting Roadway Design 
Treatments to Accommodate Bicycles, Report No. FHWA-RD~92-073, McLean, Virginia: U.S. 
Department of Transportation, Federal Highway Administration, January 1994. 

30. AASHTO. Highway Design and Operational Practices Related to Highway Safety, Washington, 
D.C.: AASHTO, 1974. 

31. AASHTO. Highway Safety Design and Operations Guide, Washington, D.C.: AASHTO, 1997. 

32. AASHTO, et al. Enhancing Highway Safety in an Age of Limited Resources, A report resulting from 
a symposium conducted by the Transportation Research Board, unpublished, November 1981. 

33. Fee, J. A., et al. Interstate System Accident Research Study 1, Washington, D.C.: U.S. Department of 
Transportation, Federal Highway Administration, October 1970. 

34. Dart, Jr., O. K. and L. Mann, Jr. "Relationship of Rural Highway Geometry to Accident Rates in 
Louisiana," Highway Research Record 312, Highway Research Board, 1970. 

35. Byington, S. R. "Interstate System Accident Research," Public Roads, Vol. 32, December 1963. 

36. Billion, C. E., and W. R. Stohner. "A Detailed Study of Accidents as Related to Highway Shoulders 
in New York State," Proceedings ofHRB, Vol. 36, Highway Research Board, 1957: 497-508. 

37. Zegeer, C. V., J. R. Stewart, F. M. Council, D. W. Reinfurt, and E. Hamdlton. "Safety Effects of 
Geometric Improvements on Horizontal Curves," Transportation Research Board 1356, 
Transportation Research Board, 1992. 

38. Graham, J. L., and D. W. Harwood. Effectiveness of Clear Recovery Zones, NCHRP Report 247, 
Washington, D.C.: Transportation Research Board, 1982. 

39. AASHTO. Roadside Design Guide, Washington, D.C.: AASHTO, 1996. 

40. Ross, H. E., D. L. Sicking, R. A. Zimmer, and J. D. Michie. Recommended Procedures for the Safety 
Performance Evaluation of Highway Features, NCHRP Report 350, Washington, D.C.: 
Transportation Research Board, 1993. 

41. AASHTO. A Manual on User Benefit Analysis of Highway and Bus-Transit Improvements, 
Washington, D.C.: AASHTO, 1977. 



108 



CHAPTER 3 
ELEMENTS OF DESIGN 

INTRODUCTION 

The alignment of a highway or street produces a great impact on the environment, the fabric 
of the community, and the highway user. The alignment is comprised of a variety of elements 
joined together to create a facility that serves the traffic in a safe and efficient manner, consistent 
with the facility's intended function. Each alignment element should complement others to 
produce a consistent, safe, and efficient design. 

The design of highways and streets within particular functional classes is treated separately 
in later chapters. Common to all classes of highways and streets are several principal elements of 
design. These include sight distance, superelevation, traveled way widening, grades, horizontal 
and vertical alignments, and other elements of geometric design. These alignment elements are 
discussed in this chapter, and, as appropriate, in the later chapters pertaining to specific highway 
functional classes. 



SIGHT DISTANCE 
General Considerations 

A driver's ability to see ahead is of the utmost importance in the safe and efficient operation 
of a vehicle on a highway. For example, on a railroad, trains are confined to a fixed path, yet a 
block signal system and trained operators are needed for safe operation. On the other hand, the 
path and speed of motor vehicles on highways and streets are subject to the control of drivers 
whose ability, training, and experience are quite varied. For safety on highways, the designer 
should provide sight distance of sufficient length that drivers can control the operation of their 
vehicles to avoid striking an unexpected object in the traveled way. Certain two-lane highways 
should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane for 
passing other vehicles without risk of a crash. Two-lane rural highways should generally provide 
such passing sight distance at frequent intervals and for substantial portions of their length. By 
contrast, it is normally of little practical value to provide passing sight distance on two-lane urban 
streets or arterials. The proportion of a highway's length with sufficient sight distance to pass 
another vehicle and interval between passing opportunities should be compatible with the design 
criteria established in the subsequent chapter pertaining to the functional classification of the 
specific highway or street. 

Four aspects of sight distance are discussed below: (1) the sight distances needed for 
stopping, which are applicable on all highways; (2) the sight distances needed for the passing of 
overtaken vehicles, applicable only on two-lane highways; (3) the sight distances needed for 
decisions at complex locations; and (4) the criteria for measuring these sight distances for use in 
design. The design of alignment and profile to provide sight distances and that satisfy the 



109 



AASHTO — Geometric Design of Highways and Streets 



applicable design criteria are described later in this chapter. The special conditions related to sight 
distances at intersections are discussed in Chapter 9. 



Stopping Sight Distance 

Sight distance is the length of the roadway ahead that is visible to the driver. The available 
sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the 
design speed to stop before reaching a stationary object in its path. Although greater lengths of 
visible roadway are desirable, the sight distance at every point along a roadway should be at least 
that needed for a below-average driver or vehicle to stop. 

Stopping sight distance is the sum of two distances: (1) the distance traversed by the vehicle 
from the instant the driver sights an object necessitating a stop to the instant the brakes are 
applied; and (2) the distance needed to stop the vehicle from the instant brake application begins. 
These are referred to as brake reaction distance and braking distance, respectively. 



Brake Reaction Time 

Brake reaction time is the interval from the instant that the driver recognizes the existence of 
an obstacle on the roadway ahead that necessitates braking to the instant that the driver actually 
applies the brakes. Under certain conditions, such as emergency situations denoted by flares or 
flashing lights, drivers accomplish these tasks almost instantly. Under most other conditions, the 
driver must not only see the object but must also recognize it as a stationary or slowly moving 
object against the background of the roadway and other objects, such as walls, fences, trees, 
poles, or bridges. Such determinations take time, and the amount of time needed varies 
considerably with the distance to the object, the visual acuity of the driver, the natural rapidity 
with which the driver reacts, the atmospheric visibility, the type and the condition of the roadway, 
and nature of the obstacle. Vehicle speed and roadway environment probably also influence 
reaction time. Normally, a driver traveling at or near the design speed is more alert than one 
traveling at a lesser speed. A driver on an urban street confronted by innumerable potential 
conflicts with parked vehicles, driveways, and cross streets is also likely to be more alert than the 
same driver on a liirdted-access facility where such conditions should be almost nonexistent. 

The study of reaction times by Johansson and Rumar (1) referred to in Chapter 2 was based 
on data from 321 drivers who expected to apply their brakes. The median reaction-time value for 
these drivers was 0.66 s, with 10 percent using 1.5 s or longer. These findings correlate with those 
of earlier studies in which alerted drivers were also evaluated. Another study (2) found 0.64 s as 
the average reaction time, while 5 percent of the drivers needed over 1 s. In a third study (3), the 
values of brake reaction time ranged from 0.4 to L7 s. In the Johansson and Rumar study (1), 
when the event that required application of the brakes was unexpected, the drivers' response 
times were found to increase by approximately 1 s or more; some reaction times were greater than 
1,5 s. This increase in reaction time substantiated earlier laboratory and road tests in which the 
conclusion was drawn that a driver who needed 0.2 to 0.3 s of reaction time under alerted 
conditions would need 1.5 s of reaction time under normal conditions. 

110 



Elements of Design 



Minimum brake reaction times for drivers could thus be at least 1.64 s and 0.64 s for alerted 
drivers as M/ell as 1 s for the unexpected event. Because the studies discussed above used simple 
prearranged signals, they represent the least complex of roadv^ay conditions. Even under these 
simple conditions, it was found that some drivers took over 3.5 s to respond. Because actual 
conditions on the highway are generally more complex than those of the studies, and because 
there is wide variation in driver reaction times, it is evident that the criterion adopted for use 
should be greater than 1.64 s. The brake reaction time used in design should be large enough to 
include the reaction times needed by nearly all drivers under most highway conditions. Both 
recent research (4) and the studies documented in the literature (1, 2^ 3) show that a 2.5-s brake 
reaction time for stopping sight situations encompasses the capabilities of most drivers, including 
those of older drivers. The recommended design criterion of 2.5 s for brake reaction time exceeds 
the 90th percentile of reaction time for all drivers and has been used in the development of 
Exhibit 3-1. 

A brake reaction time of 2.5 s is considered adequate for conditions that are more complex 
than the simple conditions used in laboratory and road tests, but it is not adequate for the most 
complex conditions encountered in actual driving. The need for greater reaction time in the most 
complex conditions encountered on the roadway, such as those found at multiphase at-grade 
intersections and at ramp terminals on through roadways, can be found later in this chapter in the 
section on "Decision Sight Distance." 



Braking Distance 

The approximate braking distance of a vehicle on a level roadway traveling at the design 
speed of the roadway may be determined from the following equation: 



Metric 


US Customary 


rf =0.039- 
a 


d = L075 - ( 3-1 ) 
a 


where: 

d = braking distance, m; 
V = design speed, km/h; 
a = deceleration rate, m/s^ 


where: 

d = braking distance, ft; 
V = design speed, mph; 
a = deceleration rate, ft/s^ 



Studies documented in the literature (4) show that most drivers decelerate at a rate greater 
than 4.5 m/s^ [14.8 ft/s^] when confronted with the need to stop for an unexpected object in the 
roadway. Approximately 90 percent of all drivers decelerate at rates greater than 3.4 m/s^ 
[1 1.2 ft/s^]. Such decelerations are within the driver's capability to stay within his or her lane and 
maintain steering control during the braking maneuver on wet surfaces. Therefore, 3.4 m/s^ 
[11.2ft/s ] (a comfortable deceleration for most drivers) is recommended as the deceleration 



111 



AASHTO — Geometric Design of Highways and Streets 





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Elements of Design 



threshold for determining stopping sight distance. Implicit in the choice of this deceleration threshold 
is the assessment that most vehicle braking systems and the tire-pavement friction levels of most 
roadways are capable of providing a deceleration of at least 3.4 m/s [11.2 ft/s]. The friction 
available on most wet pavement surfaces and the capabilities of most vehicle braking systems can 
provide braking friction that exceeds this deceleration rate. 



Design Values 

The sum of the distance traversed during the brake reaction time and the distance to brake the 
vehicle to a stop is the stopping sight distance. The computed distances for various speeds at the 
assumed conditions are shown in Exhibit 3-1 and were developed from the following equation: 



Metric 


US Customary 


d - 0.278 yr+ 0.039- 

a 


d = 147 W+ 1.075- (3-2) 
a 


where: 

t = brake reaction time, 2.5 s; 
V = design speed, km/h; 
a = deceleration rate, m/s^ 


where: 

t = brake reaction time, 2.5 s; 
V = design speed, mph; 
a = deceleration rate, ft/s^ 



Stopping sight distances exceeding those shown in Exhibit 3-1 should be used as the basis for 
design wherever practical. Use of longer stopping sight distances increases the margin of safety for all 
drivers and, in particular, for those who operate at or near the design speed. To ensure that new 
pavements will have initially, and will retain, friction coefficients comparable to the deceleration rates 
used to develop Exhibit 3-1, pavement designs should meet the criteria established in the AASHTO 
Guidelines for Skid Resistant Pavement Design (5). 

In computing and measuring stopping sight distances, the height of the driver's eye is estimated 
to be 1,080 mm [3.5 ft] and the height of the object to be seen by the driver is 600 mm [2.0 ft], 
equivalent to the taillight height of a passenger car. The application of these eye-height and object- 
height criteria is discussed further in the section on "Vertical Ahgnment" in this chapter. 



Effect of Grade on Stopping 

When a highway is on a grade, the equation for braking distance should be modified as follows: 



113 



AASHTO — Geometric Design of Highways and Streets 



Metric 



US Customary 



d = 



254 



rr a ^ 



9.81 



±G 



d = 



30 



^^ a ^ 



\\ 



32.2 



±G 



(3-3) 



In this equation, G is the percent of grade divided by 100, and the other terms are as 
previously stated. The stopping distances needed on upgrades are shorter than on level roadways; 
those on downgrades are longer. The stopping sight distances for various grades are shown in 
Exhibit 3-2. These adjusted sight distance values are computed for wet-pavement conditions 
using the same design speeds and brake reaction times used for level roadways in Exhibit 3-1. 

On nearly all roads and streets, the grade is traversed by traffic in both directions of travel, 
but the sight distance at any point on the highway generally is different in each direction, 
particularly on straight roads in roUing terrain. As a general rule, the sight distance available on 
downgrades is larger than on upgrades, more or less automatically providing the appropriate 
corrections for grade. This may explain why designers do not adjust stopping sight distance 
because of grade. Exceptions are one-way roads or streets, as on divided highways with 
independent design profiles for the two roadways. For these separate roadways, adjustments for 
grade may be needed. 



Variation for Trucks 

The recommended stopping sight distances are based on passenger car operation and do not 
explicitly consider design for truck operation. Trucks as a whole, especially the larger and heavier 
units, need longer stopping distances from a given speed than passenger vehicles. However, there 
is one factor that tends to balance the additional braking lengths for trucks with those for 
passenger cars. The truck driver is able to see substantially farther beyond vertical sight 
obstructions because of the higher position of the seat in the vehicle. Separate stopping sight 
distances for trucks and passenger cars, therefore, are not generally used in highway design. 

There is one situation in which every effort should be made to provide stopping sight 
distances greater than the design values in Exhibit 3-1. Where horizontal sight restrictions occur 
on downgrades, particularly at the ends of long downgrades where truck speeds closely approach 
or exceed those of passenger cars, the greater height of eye of the truck driver is of little value, 
even when the horizontal sight obstruction is a cut slope. Although the average truck driver tends 
to be more experienced than the average passenger car driver and quicker to recognize potential 
risks, it is desirable under such conditions to provide stopping sight distance that exceeds the 
values in Exhibits 3-1 or 3-2. 



114 



Elements of Design 



Metric | 


US Customary | 


Design 
speed 
(km/h) 


Stopping sighl 


t distance (nr 


) 


Design 
speed 
(mph) 




Stopping sight distance (ft) | 


Downgrades 


Upgrades 


Downgrades 


Upgrades j 


3% 6% 9% 


3% 


6% 


9% 


3% 


6% 9% 


3% 


6% 9% 


20 


20 20 20 


19 


18 


18 


15 


80 


82 85 


75 


74 73 


30 


32 35 35 


31 


30 


29 


20 


116 


120 126 


109 


107 104 


40 


50 50 53 


45 


44 


43 


25 


158 


165 173 


147 


143 140 


50 


66 70 74 


61 


59 


58 


30 


205 


215 227 


200 


184 179 


60 


87 92 97 


80 


77 


75 


35 


257 


271 287 


237 


229 222 


70 


110 116 124 


100 


97 


93 


40 


315 


333 354 


289 


278 269 


80 


136 144 154 


123 


118 


114 


45 


378 


400 427 


344 


331 320 


90 


164 174 187 


148 


141 


136 


50 


446 


474 507 


405 


388 375 


100 


194 207 223 


174 


167 


160 


55 


520 


553 593 


469 


450 433 


1 110 


227 243 262 


203 


194 


186 


60 


598 


638 686 


538 


515 495 


120 


263 281 304 


234 


223 


214 


65 


682 


728 785 


612 


584 561 


130 


302 323 350 


267 


254 


243 


70 


771 


825 891 


690 


658 631 












75 


866 


927 1003 


772 


736 704 












80 


965 


1035 1121 


859 


817 782 



Exhibit 3-2. Stopping Sight Distance on Grades 

Decision Sight Distance 

Stopping sight distances are usually sufficient to allow reasonably competent and alert 
drivers to come to a hurried stop under ordinary circumstances. However, these distances are 
often inadequate when drivers must make complex or instantaneous decisions, when information 
is difficult to perceive or when unexpected or unusual maneuvers are required. Limiting sight 
distances to those needed for stopping may preclude drivers from performing evasive maneuvers, 
which often involve less risk and are otherwise preferable to stopping. Even with an appropriate 
complement of standard traffic control devices in accordance with the MUTCD (6), stopping 
sight distances may not provide sufficient visibility distances for drivers to corroborate advance 
warning and to perform the appropriate maneuvers. It is evident that there are many locations 
where it would be prudent to provide longer sight distances. In these circumstances, decision 
sight distance provides the greater visibility distance that drivers need. 

Decision sight distance is the distance needed for a driver to detect an unexpected or 
otherwise difficult-to-perceive information source or condition in a roadway environment that 
may be visually cluttered, recognize the condition or its potential threat, select an appropriate 
speed and path, and initiate and complete the maneuver safely and efficiently (7). Because 
decision sight distance offers drivers additional margin for error and affords them sufficient 
length to maneuver their vehicles at the same or reduced speed, rather than to just stop, its values 
are substantially greater than stopping sight distance. 



Drivers need decision sight distances whenever there is a likelihood for error in either 
information reception, decision-making, or control actions (8). Examples of critical locations 
where these kinds of errors are likely to occur, and where it is desirable to provide decision sight 
distance include interchange and intersection locations where unusual or unexpected maneuvers 
are required, changes in cross section such as toll plazas and lane drops, and areas of concentrated 

115 



AASHTO — Geometric Design of Highways and Streets 



demand where there is apt to be "visual noise" from competing sources of information, such as 
roadway elements, traffic, traffic control devices, and advertising signs. 

The decision sight distances in Exhibit 3-3: (1) provide values for sight distances that may be 
appropriate at critical locations and (2) serve as criteria in evaluating the suitability of the 
available sight distances at these locations. Because of the additional safety and maneuvering 
space provided, it is recommended that decision sight distances be provided at critical locations 
or that critical decision points be moved to locations where sufficient decision sight distance is 
available. If it is not practical to provide decision sight distance because of horizontal or vertical 
curvature or if relocation of decision points is not practical, special attention should be given to 
the use of suitable traffic control devices for providing advance warning of the conditions that are 
likely to be encountered. 



Metric | 


US Customary | 


Desian Decision s 


iglit distance (m) 


Design . 

speed 

(mph) 




Decision sight distance (ft) | 


speed Avoidance maneuver 




Avoidance maneuver 




(km/ii) A B 


C D E 


A 


BCD 


E 


50 70 155 


145 170 195 


30 


220 


490 450 535 


620 


60 95 195 


170 205 235 


35 


275 


590 525 625 


720 


70 115 235 


200 235 275 


40 


330 


690 600 715 


825 


80 140 280 


230 270 315 


45 


395 


800 675 800 


930 


90 170 325 


270 315 360 


50 


465 


910 750 890 


1030 


100 200 370 


315 355 400 


55 


535 


1030 865 980 


1135 


110 235 420 


330 380 430 


60 


610 


1150 990 1125 


1280 


120 265 470 


360 415 470 


65 


695 


1275 1050 1220 


1365 


130 305 525 


390 450 510 


70 


780 


1410 1105 1275 


1445 






75 


875 


1545 1180 1365 


1545 






80 


970 


1685 1260 1455 


1650 


Avoidance Maneuver A 


Stop on rural road- 


-t = 3.0 s 








Avoidance Maneuver B 


Stop on urban road 


— t = 9.1 s 








Avoidance Maneuver C 


■ Speed/path/directic 


•n cliange on rural road — ^t varies between 10.2 | 


and 11.2s 












Avoidance Maneuver D 


Speed/patli/directic 


n cliange 


on suburban road— t varies between 


12.1 and 12.9 s 












Avoidance Maneuver E 


Speed/path/directio 


n change ( 


Dn urban road— t varies between 14.0 | 


and 14.5 s 










1 



Exhibit 3-3« DecisioE Sight Distance 



Decision sight distance criteria that are applicable to most situations have been developed 
from empirical data. The decision sight distances vary depending on whether the location is on a 
rural or urban road and on the type of avoidance maneuver required to negotiate the location 
properly. Exhibit 3-3 shows decision sight distance values for various situations rounded for 
design. As can be seen in the exhibit, shorter distances are generally needed for rural roads and 
for locations where a stop is the appropriate maneuver. 

For the avoidance maneuvers identified in Exhibit 3-3, the pre-maneuver time is increased 
above the brake reaction time for stopping sight distance to allow the driver additional time to 



116 



Elements of Design 



detect and recognize the roadway or traffic situation, identify alternative maneuvers, and initiate a 
response at critical locations on the highway (9). The pre-maneuver component of decision sight 
distance uses a value ranging between 3.0 and 9.1 s (10). 

The braking distance from the design speed is added to the pre-maneuver component for 
avoidance maneuvers A and B as shown in Equation (3-4). The braking component is replaced in 
avoidance maneuvers C, D, and E with a maneuver distance based on maneuver times between 
3.5 and 4.5 s, that decrease with increasing speed (9) in accordance with Equation (3-5). 

The decision sight distances for avoidance maneuvers A and B are determined as: 



Metric 


US Customary 


d = 0.278 Vr+ 0.039- 
a 


d - 1.47 W+ 1.075— (3-4) 
a 


where: 

t = pre-maneuver time, s (see 

notes in Exhibit 3-3); 
V = design speed, km/h; 
a = driver deceleration, m/s^ 


where: 

t = pre-maneuver time, s (see 

notes in Exhibit 3-3); 
V = design speed, mph; 
a = driver deceleration, ft/s^ 



The decision sight distances for avoidance maneuvers C, D, and E are determined as: 



Metric 


US Cystomary 


d = 0.278W 


d -1.47W (3-5) 


where: 

t = total pre-maneuver and 

maneuver time, s (see notes 
in Exhibit 3-3); 

V = design speed, km/h 


where: 

t = total pre-maneuver and 

maneuver time, s (see notes 
in Exhibit 3-3); 

V = design speed, mph 



In computing and measuring decision sight distances, the same 1,080-mm [3.5-ft] eye-height 
and 600-mm [2.0-ft] object-height criteria used for stopping sight distance have been adopted. 
Although drivers may have to be able to see the entire roadway situation, including the road 
surface, the rationale for the 600-mm [2.0-ft] object height is as applicable to decision sight 
distance as it is to stopping sight distance. 



117 



AASHTO — Geometric Design of Highways and Streets 



Passing Sight Distance for Two-Lane Highways 

Criteria for Desigii 

Most roads and many streets are two-lane, two-way highways on which vehicles frequently 
overtake slower moving vehicles. Passing maneuvers in which faster vehicles move ahead of 
slower vehicles must be accomplished on lanes regularly used by opposing traffic. If passing is to 
be accomplished safely, the passing driver should be able to see a sufficient distance ahead, clear 
of traffic, to complete the passing maneuver without cutting off the passed vehicle before meeting 
an opposing vehicle that appears during the maneuver. When appropriate, the driver can return to 
the right lane without completing the pass if he or she sees opposing traffic is too close when the 
maneuver is only partially completed. Many passing maneuvers are accomplished without the 
driver being able to see any potentially conflicting vehicle at the beginning of the maneuver, but 
design should not be based on such maneuvers. Because many cautious drivers would not attempt 
to pass under such conditions, design on this basis would reduce the usefulness of the highway. 
An alternative to providing passing sight distance is found later in this chapter in the section on 
"Passing Lanes." 

Passing sight distance for use in design should be determined on the basis of the length 
needed to complete normal passing maneuvers in which the passing driver can determine that 
there are no potentially conflicting vehicles ahead before beginning the maneuver. While there 
may be occasions to consider multiple passings, where two or more vehicles pass or are passed, it 
is not practical to assume such conditions in developing minimum design criteria. Instead, sight 
distance should be determined for a single vehicle passing a single vehicle. Longer sight distances 
occur in design and such locations can accommodate an occasional multiple passing. 

Minimum passing sight distances for design of two-lane highways incorporate certain 
assumptions about driver behavior. Actual driver behavior in passing maneuvers varies widely. 
To accommodate these variations in driver behavior, the design criteria for passing sight distance 
should accommodate the behavior of a high percentage of drivers, rather than just the average 
driver. The following assumptions are made concerning driver behavior in passing maneuvers: 

L The overtaken vehicle travels at uniform speed. 

2. The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a 
passing section. 

3. When the passing section is reached, the passing driver needs a short period of time to 
perceive the clear passing section and to react to start his or her maneuver. 

4. Passing is accomplished under what may be termed a delayed start and a hurried return 
in the face of opposing traffic. The passing vehicle accelerates during the maneuver, 
and its average speed during the occupancy of the left lane is 15 km/h [10 mph] higher 
than that of the overtaken vehicle. 

5. When the passing vehicle returns to its lane, there is a suitable clearance length between 
it and an oncoming vehicle in the other lane. 

Some drivers accelerate at the beginning of a passing maneuver to an appreciably higher 
speed and then continue at a uniform speed until the maneuver is completed. Many drivers 

118 



Elements of Design 



accelerate at a fairly high rate until just beyond the vehicle being passed and then complete the 
maneuver either without further acceleration or at reduced speed. For simplicity, such 
extraordinary maneuvers are ignored and passing distances are developed with the use of 
observed speeds and times that fit the practices of a high percentage of drivers. 

The minimum passing sight distance for two-lane highways is determined as the sum of the 
following four distances (shown in Exhibit 3-4): 

® di^ — Distance traversed during perception and reaction time and during the initial 

acceleration to the point of encroachment on the left lane. 
® d2— Distance traveled while the passing vehicle occupies the left lane. 
® ds — Distance between the passing vehicle at the end of its maneuver and the opposing 

vehicle. 
® d4 — Distance traversed by an opposing vehicle for two-thirds of the time the passing 

vehicle occupies the left lane, or 2/3 of d2 above. 





Opposing vehicle appears 
when passing vehicle 

FIRST PHASE «-''-^-«^-— | 

A B 




S 

b::3ib 


1 




d^ 


5/3 d^ 


SECOND PHASE 












— 






s- ffl:::-:::::;! 


k 




d, 




2/Z d, i 


^4 











Exhibit 3-4. Elements of Passing Sight Distance for Two-Lane Highways 



Various distances for the components of passing maneuvers, based on extensive field 
observations of driver behavior (11) are presented for four passing speed groups in Exhibit 3-5. 
Time and distance values were determined in relation to the average speed of the passing vehicle. 
The speeds of the overtaken vehicles were approximately 15 km/h [10 mph] less than the speeds 
of the passing vehicles. 

Very little change was noted in the passing practices of drivers in a restudy of three of the 
original sections despite increased vehicle performance capabilities. A later study (12) of vehicle 
passing performance on two-lane highways produced a different set of passing sight distance 
values. These values were subsequently reviewed (13) to evaluate minimum passing sight 
distances. This evaluation reported the total passing sight distances as seen in Exhibit 3-5 are 
greater than those determined in subsequent studies for all speeds except 110 km/h [70 mph]. 
Thus, the minimum passing sight distances presented in Exhibit 3-7 are generally conservative for 
modem vehicles and are used below. 



119 



AASHTO — Geometric Design of Highways and Streets 







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720 



Elements of Design 



Initial maneuver distance (di). The initial maneuver period has two components, a time for 
perception and reaction, and an interval during which the driver brings the vehicle from the 
trailing speed to the point of encroachment on the left or passing lane. To a great extent these two 
periods overlap. As a passing section of highway comes into the view of a driver desiring to pass, 
the driver may begin to accelerate and maneuver his or her vehicle toward the centerline of the 
highway while deciding whether or not to pass. Studies show that the average passing vehicle 
accelerates at less than its maximum potential, indicating that the initial maneuver period contains 
an element of time for perception and reaction. However, some drivers may remain in normal 
lane position while deciding to pass. The exact position of the vehicle during initial maneuver is 
unimportant because the differences in resulting passing distances are insignificant. 

The acceleration rate obtained from the passing study data in the first three speed groups 
during the initial maneuver period varied from 2.25 to 2.37 km/h/s [1.41 to 1.47 mph/s]; the 
average time varied from 3.7 to 4.3 s; and the average passing speeds were 56.2, 70.0, and 
84.5 km/h [34.9, 43.8, and 52.6 mph]. For the 96 to 100 km/h [60 to 70 mph] group, on the basis 
of extrapolated data, the average acceleration was assumed to be 2.41 km/h/s [1.50 mph/s]; the 
maneuver time, 4.5 s; and the average speed, 99.8 km/h [62.0 mph]. 

The distance di traveled during the initial maneuver period is computed with the following 
equation: 



Metric | 


US Cystomary | 


d, - 0.278 r, 


f «0 

V - m + — - 

[ 2j 




d, =147 1, 


V - m + — - 

2 , 


(3-6) 


where: 

tj = time of initial maneuver, s; 
a = average acceleration, 

km/h/s; 
V = average speed of passing 

vehicle, km/h; 
m = difference in speed of 

passed vehicle and passing 

vehicle, km/h 


where: 

tj = time of initial maneuver, s; 
a = average acceleration, 

mph/s; 
v = average speed of passing 

vehicle, mph; 
m = difference in speed of 

passed vehicle and 

passing vehicle, mph 



The acceleration, time, and distance traveled during the initial maneuver periods in passing 
are given in Exhibit 3-5. The d] line in Exhibit 3-6 shows the distance plotted against the average 
speed of the passing vehicle. 

Distance while passing vehicle occupies left lane (d2). Passing vehicles were found in the 
study to occupy the left lane from 9.3 to 10.4 s. The distance d2 traveled in the left lane by the 
passing vehicle is computed with the following equation: 



727 



AASHTO — Geometric Design of Highways and Streets 



y@trio 


US Customary 


^2 = O.llSvt^ 


d^ ^lAlvt^ (3-7) 


where: 

ta = time passing vehicle 

occupies the left lane, s; 

V = average speed of passing 
vehicle, km/h 


where: 

X2 = tinne passing vehicle 

occupies the left lane, s; 

V = average speed of passing 
vehicle, mph 



The time and distance traveled while the passing vehicle occupies the left lane are given in 
Exhibit 3-5. Distances are plotted against average passing speeds as curve d2 in Exhibit 3-6. 

Clearance length (ds). The clearance length between the opposing and passing vehicles at 
the end of the passing maneuvers was found in the passing study to vary from 30 to 75 m [100 to 
250 ft]. This length, adjusted somewhat for practical consistency, is shown as the clearance length 
ds in Exhibits 3-5 and 3-6. 

Distance traversed by an opposing vehicle (dd. Passing sight distance includes the 
distance traversed by an opposing vehicle during the passing maneuver, to minimize the chance 
that a passing vehicle will meet an opposing vehicle while in the left lane. Conservatively, this 
distance should be the distance traversed by an opposing vehicle during the entire time it takes to 
pass or during the time the passing vehicle is in the left lane, but such distance is questionably 
long. During the first phase of the passing maneuver, the passing vehicle has not yet pulled 
abreast of the vehicle being passed, and even though the passing vehicle occupies the left lane, its 
driver can return to the right lane if an opposing vehicle is seen. It is unnecessary to include this 
trailing time interval in computing the distance traversed by an opposing vehicle. This time 
interval, which can be computed from the relative positions of passing and passed vehicle, is 
about one-third the time the passing vehicle occupies the left lane, so that the passing sight 
distance element for the opposing vehicle is the distance it traverses during two-thirds of the time 
the passing vehicle occupies the left lane. The opposing vehicle is assumed to be traveling at the 
same speed as the passing vehicle, so d4 = 2d2/3. The distance d4 is shown in Exhibits 3-5 and 
3-6. 



Design Values 

The "total" curve in Exhibit 3-6 is determined by the sum of the elements di through d4. For 
each passing speed, this total curve indicates the minimum passing sight distance for a vehicle to 
pass another vehicle travehng 15 km/h [10 mph] slower, in the face of an opposing vehicle 
traveling at the same speed as the passing vehicle. On determination of a likely and logical 
relation between average passing speed and the highway design speed, these distances can be 
used to express the minimum passing sight distance needed for design purposes. 



722 



Elements of Design 



METRIC 



1200 



£-1000 

9 
O 

C 

I 800 

f 

o 600 



J 200 





















rotal = d, + dj 


+ d3 + d4 


y 










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<u 


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da 



Average speed ^ Passiiif v9hicli» (km/h) 



US CUSTOMARY 



3500 






1 2000 















/ 




: 






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tai = di*da 


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SO 



60 



Average «j;>9«4 - ^as^big v^^Ua (mph) 



Exhibit 3-6o Total Passing Sight Distance and Its Components — Two-Lane Highways 



The ranges of speeds of the passed and passing vehicles are affected by traffic volume. 
When traffic volume is low (level-of-service A), there are few vehicles that need to be passed, but 
as the volume increases (level-of-service D or lower) there are few, if any, passing opportunities. 
The speed of the passed vehicle has been assumed to be the average running speed at a traffic 
volume near capacity. The speed of the passing vehicle is assumed to be 15 km/h [10 mph] 
greater. The assumed speeds for passing vehicles in Exhibit 3-7 represent the likely passing 
speeds on two-lane highways. Passing sight distances for these passing speeds would 
accommodate a majority of the desired passing maneuvers and correspond to the total curve in 
Exhibit 3-6. The values in the last column of Exhibit 3-7 are design values for minimum passing 
sight distance. In designing a highway these distances should be exceeded as much as practical, 
and passing sections should be provided as often as can be done at reasonable cost to provide as 
many passing opportunities as practical. 



These minimum passing sight distances for design should not be confused with other 
distances used as the warrants for placing no-passing zone pavement markings on completed 
highways. Such values as shown in the MUTCD (6) are substantially less than design distances 

123 



AASHTO — Geometric Design of Highways and Streets 







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724 



Elements of Design 



and are derived for traffic operating-control needs that are based on different assumptions from 
those for highway design. 



Effect ©f Grade on Passing Sight Distance 

Appreciable grades affect the sight distance needed for passing. Passing is easier for the 
vehicle traveling dov^ngrade because the overtaking vehicle can accelerate more rapidly than on 
the level and thus can reduce the time of passing. However, the overtaken vehicle can also 
accelerate easily so that a situation akin to a racing contest may result. 

The sight distances needed to permit vehicles traveling upgrade to pass safely are greater 
than those needed on level roads because of reduced acceleration of the passing vehicle (which 
increases the time of passing) and the likelihood that opposing traffic may speed up (which 
increases the distance traveled by an opposing vehicle during the passing maneuver). 
Compensating for this somewhat are the factors that the passed vehicle frequently is a truck that 
usually loses some speed on appreciable upgrades and that many drivers are aware of the greater 
distances needed for passing upgrade compared with level conditions. 

If passing maneuvers are to be performed on upgrades under the same assumptions about the 
behavior of the passing and passed vehicles discussed above, the passing sight distance should be 
greater than the derived design values. Specific adjustments for design use are unavailable, but 
the designer should recognize the desirability of exceeding the values shown in Exhibit 3-7. 



Frequency and Lengtti of Passing Sections 

Sight distance adequate for passing should be encountered frequently on two-lane highways. 
Each passing section along a length of roadway with sight distance ahead equal to or greater than 
the minimum passing sight distance should be as long as practical. The frequency and length of 
passing sections for highways depend, principally on the topography, the design speed of 
highway, and the cost; for streets, the spacing of intersections is the principal consideration. 

It is not practical to directly indicate the frequency with which passing sections should be 
provided on two-lane highways due to the physical and cost limitations. During the course of 
normal design, passing sections are provided on almost all highways and selected streets, but the 
designer's appreciation of their importance and a studied attempt to provide them can usually 
ensure others at little or no additional cost. In steep mountainous terrain, it may be more 
economical to build intermittent four-lane sections or passing lanes with stopping sight distance 
on some two-lane highways, in lieu of two-lane sections with passing sight distance. Alternatives 
are discussed later in this chapter in the section on "Passing Lanes." 

The passing sight distances shown in Exhibit 3-7 are sufficient for a single or isolated pass 
only. Designs with infrequent passing sections will not assure that opportunities for passing are 
available. Even on low-volume roadways, a driver desiring to pass may, on reaching the passing 



125 



AASHTO — Geometric Design of Highways and Streets 



section, find vehicles in the opposing lane and thus be unable to use the passing section or at least 
may not be able to begin to pass at once. 

The importance of frequent passing sections is illustrated by their effect on the level of 
service of a two-lane, two-way highway. The procedures in the Highway Capacity Manual (14) to 
analyze two-lane, two-way highways base the level-of-service criteria on two measures of 
effectiveness — percent time spent following and average travel speed. Both of these criteria are 
affected by the lack of passing opportunities. The HCM procedures show, for example, up to a 19 
percent increase in the percent time spent following when the directional split is 50/50 and no- 
passing zones comprise 40 percent of the analysis length compared to a highway with similar 
traffic volumes and no sight restrictions. The effect of restricted passing sight distance is even 
more severe for unbalanced flow and where the no-passing zones comprise more than 40 percent 
of the length. 

There is a similar effect on the average travel speed. As the percent of no-passing zones 
increases, there is an increased reduction in the average travel speed for the same demand flow 
rate. For example, a demand flow rate of 800 passenger cars per hour incurs a reduction of 
3.1 km/h (1.9 mph) when no-passing zones comprise 40 percent of the analysis length compared 
to no reduction in speed on a route with unrestricted passing. 

The HCM procedures indicate another possible criterion for passing sight distance design on 
two-lane highways that are several miles or more in length. The available passing sight distances 
along this length can be summarized to show the percentage of length with greater-than-minimum 
passing sight distance. Analysis of capacity related to this percentage would indicate whether or 
not alignment and profile adjustments are needed to accommodate the design hourly volume 
(DHV). When highway sight distances are analyzed over the whole range of lengths within which 
passing maneuvers are made, a new design criterion may be evaluated. Where high traffic 
volumes are expected on a highway and a high level of service is to be maintained, frequent or 
nearly continuous passing sight distances should be provided. 



Sight Distance for Multilane Highways 

It is not necessary to consider passing sight distance on highways or streets that have two or 
more traffic lanes in each direction of travel. Passing maneuvers on multilane roadways are 
expected to occur within the limits of the traveled way for each direction of travel. Thus, passing 
maneuvers that involve crossing the centerline of four-lane undivided roadways or crossing the 
median of four-lane roadways should be prohibited. 

Multilane roadways should have continuously adequate stopping sight distance, with 
greater-than-design sight distances preferred. Design criteria for stopping sight distance vary with 
vehicle speed and are discussed in detail at the beginning of this chapter. 



726 



Elements of Design 



Criteria for Measuring Sight Distance 

Sight distance is the distance along a roadway throughout which an object of specified 
height is continuously visible to the driver. This distance is dependent on the height of the 
driver's eye above the road surface, the specified object height above the road surface, and the 
height and lateral position of sight obstructions within the driver's line of sight. 



Height of Driver's Eye 

For sight distance calculations for passenger vehicles, the height of the driver's eye is 
considered to be 1,080 mm [3.5 ft] above the road surface. This value is based on a study (4) 
found that average vehicle heights have decreased to 1,300 mm [4.25 ft] with a comparable 
decrease in average eye heights to 1,080 mm [3.5 ft]. Because of various factors that appear to 
place practical limits on further decreases in passenger car heights and the relatively small 
increases in the lengths of vertical curves that would result from further changes that do occur, 
1,080 mm [3.5 ft] is considered to be the appropriate height of driver's eye for measuring both 
stopping and passing sight distances. For large trucks, the driver eye height ranges from 1,800 to 
2,400 mm [5.9 to 7.9 ft]. The recommended value of truck driver eye height for design is 
2,330 mm [7.6 ft] above the roadway surface. 



Height of Object 

For stopping sight distance calculations, the height of object is considered to be 600 mm 
[2.0 ft] above the road surface. For passing sight distance calculations, the height of object is 
considered to be 1,080 mm [3.5 ft] above the road surface. 

Stopping sight distance object The basis for selection of a 600-mm [2.0-ft] object height 
was largely an arbitrary rationalization of the size of object that might potentially be encountered 
in the road and of a driver's ability to perceive and react to such situations. It is considered that an 
object 600 mm [2.0 ft] high is representative of an object that involves risk to drivers and can be 
recognized by a driver in time to stop before reaching it. Using object heights of less than 
600 mm [2.0 ft] for stopping sight distance calculations would result in longer crest vertical 
curves without documented safety benefits (4). Object height of less than 600 mm [2.0 ft] could 
substantially increase construction costs because additional excavation would be needed to 
provide the longer crest vertical curves. It is also doubtful that the driver's ability to perceive 
situations involving risk of colHsions would be increased because recommended stopping sight 
distances for high-speed design are beyond most drivers' capabilities to detect small objects (4). 

Passing sight distance object. An object height of 1,080 mm [3.5 ft] is adopted for passing 
sight distance. This object height is based on a vehicle height of 1,330 mm [4.35 ft], which 
represents the 15th percentile of vehicle heights in the current passenger car population, less an 
allowance of 250 mm [0.82 ft], which represents a near-maximum value for the portion of the 
vehicle height that needs to be visible for another driver to recognize a vehicle as such (15). 
Passing sight distances calculated on this basis are also considered adequate for night conditions 

127 



AASHTO — Geometric Design of Highways and Streets 



because headlight beams of an opposing vehicle generally can be seen from a greater distance 
than a vehicle can be recognized in the daytime. The choice of an object height equal to the driver 
eye height makes passing sight distance design reciprocal (i.e., when the driver of the passing 
vehicle can see the opposing vehicle, the driver of the opposing vehicle can also see the passing 
vehicle). 

Sight Obstructions 

On a tangent roadway, the obstruction that limits the driver's sight distance is the road 
surface at some point on a crest vertical curve. On horizontal curves, the obstruction that limits 
the driver's sight distance may be the road surface at some point on a crest vertical curve, or it 
may be some physical feature outside of the traveled way, such as a longitudinal barrier, a bridge- 
approach fill slope, a tree, foliage, or the backslope of a cut section. Accordingly, all highway 
construction plans should be checked in both the vertical and horizontal plane for sight distance 
obstructions. 

Measuring and Recording Sight Distance on Plans 

The design of horizontal alignment and vertical profile using sight distance and other criteria 
is addressed later in this chapter, including the detailed design of horizontal and vertical curves. 
Sight distance should be considered in the preliminary stages of design when both the horizontal 
and vertical alignment are still subject to adjustment. By determining the available sight distances 
graphically on the plans and recording them at frequent intervals, the designer can appraise the 
overall layout and effect a more balanced design by minor adjustments in the plan or profile. 
Methods for scaling sight distances on plans are demonstrated in Exhibit 3-8, which also shows a 
typical sight distance record that would be shown on the final plans. 

Because the view of the highway ahead may change rapidly in a short distance, it is desirable 
to measure and record sight distance for both directions of travel at each station. Both horizontal 
and vertical sight distances should be measured and the shorter lengths recorded. In the case of a 
two-lane highway, passing sight distance should be measured and recorded in addition to 
stopping sight distance. 

Sight distance charts such as those in Exhibit 3-74 and 3-77 may be used to establish 
minimum lengths of vertical curves. Charts similar to Exhibit 3-57 are useful for determining the 
radius of horizontal curve or the lateral offset from the traveled way needed to provide the design 
sight distance. Once the horizontal and vertical alignments are tentatively established, the most 
practical means of examining sight distances along the proposed highway is by direct scaling on 
the plans. 

Horizontal sight distance on the inside of a curve is limited by obstructions such as 
buildings, hedges, wooded areas, high ground, or other topographic features. These are generally 
plotted on the plans. Horizontal sight is measured with a straightedge, as indicated in the upper 
left portion of Exhibit 3-8. The cut slope obstruction is shown on the worksheets by a line 



128 



Elements of Design 



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EXAMPLE aCHT DISTANCE RECORD 



Exhibit 3-8. Scaling and Recording Sight Distances on Plans 



129 



AASHTO — Geometric Design of Highways and Streets 



representing the proposed excavation slope at a point 840 mm [2.75 ft] above the road surface 
(i.e., the approximate average of 1,080 mm and 600 mm [3.5 ft and 2.0 ft]) for stopping sight 
distance and at a point about 1,080 mm [3.5 ft] above the road surface for passing sight distance. 
The position of this Hne with respect to the centerline may be scaled from the plotted highway 
cross sections. Preferably, the stopping sight distance should be measured between points on one 
traffic lane, and passing sight distance from the middle of one lane to the middle of the other lane. 

Such refinement on two-lane highways generally is not necessary and measurement of sight 
distance along the centerline or traveled way edge is suitable. Where there are changes of grade 
coincident with horizontal curves that have sight-limiting cut slopes on the inside, the line-of- 
sight intercepts the slope at a level either lower or higher than the assumed average height. In 
measuring sight distance, the error in the use of the assumed 840 or 1,080 mm [2.75 or 3.5 ft] 
height usually can be ignored. 

Vertical sight distance may be scaled from a plotted profile by the method illustrated at the 
right center of Exhibit 3-8. A transparent strip with parallel edges 1,080 mm [3.5 ft] apart and 
with a scratched line 600 mm [2.0 ft] from the upper edge, in accordance with the vertical scale, 
is a useful tool. The lower edge of the strip is placed on the station from which the vertical sight 
distance is desired, and the strip is pivoted about this point until the upper edge is tangent to the 
profile. The distance between the initial station and the station on the profile intersected by the 
600 mm [2.0 ft] line is the stopping sight distance. The distance between the initial station and the 
station on the profile intersected by the lower edge of the strip is the passing sight distance. 

A simple sight distance record is shown in the lower part of Exhibit 3-8. Sight distances in 
both directions are indicated by arrows and figures at each station on the plan and profile sheet of 
the proposed highway. To avoid the extra work of measuring unusually long sight distances that 
may occasionally be found, a selected maximum value may be recorded. In the example shown, 
all sight distances of more than 1,000 m [3,000 ft] are recorded as 1,000 m4- [3,000 ft-f], and 
where this occurs for several consecutive stations, the intermediate values are omitted. Sight 
distances less than 500 m [1,500 ft] may be scaled to the nearest 10 m [50 ft] and those greater 
than 500 m [1,500 ft] to the nearest 50 m [100 ft]. The available sight distances along a proposed 
highway also may be shown by other methods. Several States use a sight distance graph, plotted 
in conjunction with the plan and profile of the highway, as a means of demonstrating sight 
distances. Sight distances can also be easily determined where plans and profiles are drawn using 
computer-aided design and drafting (CADD) systems. 

Sight distance records for two-lane highways may be used effectively to tentatively 
determine the marking of no-passing zones in accordance with criteria given in the MUTCD (6). 
Marking of such zones is an operational rather than a design problem. No-passing zones thus 
established serve as a guide for markings when the highway is completed. The zones so 
determined should be checked and adjusted by field measurements before actual markings are 
placed. 

Sight distance records also are useful on two-lane highways for determining the percentage 
of length of highway on which sight distance is restricted to less than the passing minimum, 
which is important in evaluating capacity. With recorded sight distances, as in the lower part of 

130 



Elements of Design 



Exhibit 3-8, it is a simple process to determine the percentage of length of highway with a given 
sight distance or greater. 



HORIZONTAL ALIGNMENT 
Theoretical Considerations 



For balance in highway design all geometric elements should, as far as economically 
practical, be designed to provide safe, continuous operation at a speed likely to be observed under 
the normal conditions for that roadway. For the most part, this can be achieved through the use of 
design speed as an overall design control. The design of roadway curves should be based on an 
appropriate relationship between design speed and curvature and on their joint relationships with 
superelevation and side friction. Although these relationships stem from the laws of mechanics, 
the actual values for use in design depend on practical limits and factors determined more or less 
empirically over the range of variables involved. These limits and factors are explained in the 
following discussion, as they relate to the determination of logical controls for roadway curve 
design. 

When a vehicle moves in a circular path, it undergoes a centripetal acceleration that acts 
toward the center of curvature. This acceleration is sustained by a component of the vehicle's 
weight related to the roadway superelevation, by the side friction developed between the vehicle's 
tires and the pavement surface, or by a combination of the two. As a matter of conceptual 
convenience, centripetal acceleration is sometimes equated to centrifugal force. However, this is 
an imaginary force that motorists believe is pushing them outward while cornering when, in fact, 
they are truly feeling the vehicle being accelerated in an inward direction. The term "centripetal 
acceleration" and its equivalent in horizontal curve design, "lateral acceleration," are used in this 
policy as they are fundamentally correct. 

From the laws of mechanics, the basic formula that governs vehicle operation on a curve is: 



Metric 


US Custoniary 


O.Ok-f/ v^ 0.0079y' V^ 
1-O.Ok/ gR R 121 R 


O.Ok-h/ v^ 0.067y^ v 
1-O.Ok/ gR R ' 15R ^^"®^ 


where: 

e = rate of roadway superelevation, 

percent; 
f = side friction (demand) factor; 

V = vehicle speed, m/s; 

g = gravitational constant, 9.81 m/s^; 

V = vehicle speed, km/h; 
R = radius of curve, m 


where: 

e = rate of roadway superelevation, 

percent; 
f = side friction (demand) factor; 

V = vehicle speed, ft/s; 

g = gravitational constant, 32.2 ft/s^; 

V = vehicle speed, mph; 
R = radius of curve, ft 



Equation (3-8), which models the moving vehicle as a point mass, is often referred to as the basic 
curve formula. 

131 



AASHTO — Geometric Design of Highways and Streets 



When a vehicle travels at constant speed on a curve superelevated so that the f value is zero, 
the centripetal acceleration is sustained by a component of the vehicle's weight and, theoretically, 
no steering force is needed. A vehicle traveling faster or slower than the balance speed develops 
tire friction as steering effort is applied to prevent movement to the outside or to the inside of the 
curve. On nonsuperelevated curves, travel at different speeds is also possible by utilizing 
appropriate amounts of side friction to sustain the varying centripetal acceleration. 



General Considerations 

From accumulated research and experience, limiting values for superelevation rate (emax) and 
side friction demand (fmax) have been established for curve design. Using these established 
limiting values in the basic curve formula permits determination of a minimum curve radius for 
various design speeds. Use of curves with radii larger than this minimum allows superelevation, 
side friction, or both to have values below their respective limits. The amount by which each 
factor is below its respective limit is chosen to provide an equitable contribution of each factor 
toward sustaining the resultant centripetal acceleration. The methods used to achieve this equity 
for different design situations are discussed below. 



Superelevation 

There are practical upper limits to the rate of superelevation on a horizontal curve. These 
limits relate to considerations of climate, constructability, adjacent land use, and the frequency of 
slow-moving vehicles. Where snow and ice are a factor, the rate of superelevation should not 
exceed the rate on which vehicles standing or traveling slowly would slide toward the center of 
the curve when the pavement is icy. At higher speeds, the phenomenon of partial hydroplaning 
can occur on curves with poor drainage that allows water to build up on the pavement surface. 
Skidding occurs, usually at the rear wheels, when the lubricating effect of the water film reduces 
the available lateral friction below the friction demand for cornering. When travelling slowly 
around a curve with high superelevation, negative lateral forces develop and the vehicle is held in 
the proper path only when the driver steers up the slope or against the direction of the horizontal 
curve. Steering in this direction seems unnatural to the driver and may explain the difficulty of 
driving on roads where the superelevation is in excess of that needed for travel at normal speeds. 
Such high rates of superelevation are undesirable on high-volume roads, as in urban and suburban 
areas, where there are numerous occasions when vehicle speeds may be considerably reduced 
because of the volume of traffic or other conditions. 

Some vehicles have high centers of gravity and some passenger cars are loosely suspended 
on their axles. When these vehicles travel slowly on steep cross slopes, a high percentage of their 
weight is carried by the inner tires. A vehicle can roll over if this condition becomes extreme. 

A discussion of these considerations and the rationale used to establish an appropriate 
maximum rate of superelevation for design of horizontal curves is provided in the subsequent 
section on "Maximum Superelevation Rates." 



132 



Elements of Design 



Side Friction Factor 

The side friction factor represents the vehicle's need for side friction, also called the side 
friction demand; it also represents the lateral acceleration af that acts on the vehicle. This 
acceleration can be computed as the product of the side friction demand factor f and the 
gravitational constant g (i.e., af = fg). It should be noted that the lateral acceleration actually 
experienced by vehicle occupants tends to be slightly larger than predicted by the product fg due 
to vehicle body roll angle. 

With the wide variation in vehicle speeds on curves, there usually is an unbalanced force 
whether the curve is superelevated or not. This force results in tire side thrust, which is 
counterbalanced by friction between the tires and the pavement surface. This frictional 
counterforce is developed by distortion of the contact area of the tire. 

The coefficient of friction f is the friction force divided by the component of the weight 
perpendicular to the pavement surface and is expressed as a simplification of the basic curve 
formula shown as Equation (3-8). The value of the product ef in this formula is always small. As 
a result, the 1-O.Olef term is nearly equal to 1.0 and is normally omitted in roadway design. 
Omission of this term yields the following basic side friction equation: 



Metric 



US Customary 



f=— 0.0k 

UlR 



y2 

/ = ™ 0.0k ( 3-9 ) 

15R 



This equation is referred to as the simplified curve formula and yields slightly larger (and, 
thus, more conservative) estimates of friction demand than would be obtained using the basic 
curve formula. 

The coefficient f has been called lateral ratio, cornering ratio, unbalanced centrifugal ratio, 
friction factor, and side friction factor. Because of its widespread use, the last term is used in this 
discussion. The upper limit of the side friction factor is the point at which the tire would begin to 
skid; this is known as the point of impending skid. Because highway curves are designed to avoid 
skidding conditions with a margin of safety, the f values used in design should be substantially 
less than the coefficient of friction at impending skid. 

The side friction factor at impending skid depends on a number of other factors, among 
which the most important are the speed of the vehicle, the type and condition of the roadway 
surface, and the type and condition of the vehicle tires. Different observers have recorded 
different maximum side friction factors at the same speeds for pavements of similar composition, 
and logically so, because of the inherent variability in pavement texture, weather conditions, and 
tire condition. In general, studies show that the maximum side friction factors developed between 
new tires and wet concrete pavements range from about 0.5 at 30 km/h [20 mph] to 
approximately 0.35 at 1(X) km/h [60 mph]. For normal wet concrete pavements and smooth tires 



133 



AASHTO — Geometric Design of Highways and Streets 



the maximum side friction factor at impeding skid is about 0.35 at 70 km/h [45 mph]. In all cases, 
the studies show a decrease in friction values as speeds increase (I69 1?^ 18). 

Horizontal curves should not be designed directly on the basis of the maximum available 
side friction factor. Rather, the maximum side friction factor used in design should be that portion 
of the maximum available side friction that can be used with comfort and safety by the vast 
majority of drivers. Side friction levels that represent pavements that are glazed, bleeding, or 
otherwise lacking in reasonable skid-resistant properties should not control design because such 
conditions are avoidable and geometric design should be based on acceptable surface conditions 
attainable at reasonable cost. 

A key consideration in selecting maximum side friction factors for use in design is the level 
of centripetal or lateral acceleration that is sufficient to cause drivers to experience a feeling of 
discomfort and to react instinctively to avoid higher speed. The speed on a curve at which 
discomfort due to the lateral acceleration is evident to drivers has been accepted as a design 
control for the maximum side friction factor. At lower nonuniform running speeds, which are 
typical in urban areas, drivers are more tolerant of discomfort, thus permitting employment of an 
increased amount of side friction for use in design of horizontal curves. 

The ball-bank indicator has been widely used by research groups, local agencies, and 
highway departments as a uniform measure of lateral acceleration to set speeds on curves that 
avoid driver discomfort. It consists of a steel ball in a sealed glass tube; except for the damping 
effect of the liquid in the tube, the ball is free to roll. Its simplicity of construction and operation 
has led to widespread acceptance as a guide for determination of appropriate curve speeds. With 
such a device mounted in a vehicle in motion, the ball-bank reading at any time is indicative of 
the combined effect of body roll, lateral acceleration angle, and superelevation as shown in 
Exhibit 3-9. 




<x = Ball Sank Indicator ongte 

p = Body roll angle 

^ = Supereievotion ongle 

= Centripeto) acceleration angle 



Exhibit 3-9» Geometry for Ball-Bank Indicator 



The centripetal acceleration developed as a vehicle travels at uniform speed on a curve 
causes the ball to roll out to a fixed angle position as shown in Exhibit 3-9. A correction should 
be made for that portion of the force taken up in the small body roll angle. The indicated side 
force perceived by the vehicle occupants is thus on the order of F - tan (a-p). 

134 



Elements of Design 



In a series of definitive tests (18), it was concluded that speeds on curves ttiat avoid driver 
discomfort are indicated by bail-bank readings of 14 degrees for speeds of 30 km/h [20 mph] or 
less, 12 degrees for speeds of 40 and 50 km/h [25 and 30 mph], and 10 degrees for speeds of 55 
through 80 km/h [35 through 50 mph]. These ball-bank readings are indicative of side friction 
factors of 0.21, 0.18, and 0.15, respectively, for the test body roll angles and provide ample 
margin of safety against skidding. 

From other tests (19), a maximum side friction factor of 0.16 for speeds up to 100 km/h 
[60 mph] was recommended. For higher speeds, the incremental reduction of this factor was 
recommended. Speed studies on the Pennsylvania Turnpike (17) led to a conclusion that the side 
friction factor should not exceed 0.10 for design speeds of 110 km/h [70 mph] and higher. A 
recent study (20) re-examined previously published findings and analyzed new data collected at 
numerous horizontal curves. The side friction demand factors developed in that study are 
generally consistent with the side friction factors reported above. 

An electronic accelerometer provides an alternative to the ball-bank indicator for use in 
determining advisory speeds for horizontal curves and ramps. An accelerometer is a gravity- 
sensitive electronic device that can measure the lateral forces and accelerations that drivers 
experience while traversing a highway curve (65). 

It should be recognized that other factors influence driver speed choice under conditions of 
high friction demand. Swerving becomes perceptible, drift angle increases, and increased steering 
effort is needed to avoid involuntary lane line violations. Under these conditions, the cone of 
vision narrows and is accompanied by an increasing sense of concentration and intensity 
considered undesirable by most drivers. These factors are more apparent to a driver under 
open-road conditions. 

Where practical, the maximum side friction factors used in design should be conservative for 
dry pavements and should provide an ample margin of safety against skidding on pavements that 
are wet as well as ice or snow covered. The need to provide skid-resistant pavement surfacing for 
these conditions cannot be overemphasized because superimposed on the frictional demands 
resulting from roadway geometry are those that result from driving maneuvers such as braking, 
sudden lane changes, and minor changes in direction within a lane. In these short-term 
maneuvers, high friction demand can exist but the discomfort threshold may not be perceived in 
time for the driver to take corrective action. 

Exhibit 3-10 summarizes the findings of the cited tests relating to side friction factors 
recommended for curve design. Although some variation in the test results is noted, all are in 
agreement that the side friction factor should be lower for high-speed design than for low -speed 
design. The subsequent sections in this chapter on "Design for Rural Highways, Urban Freeways, 
and High-Speed Urban Streets'* and "Design for Low-Speed Urban Streets" should be referred to 
for the values of the side friction factor recommended for use in horizontal curve design. Exhibit 
3-11 compares the friction factors assumed for the three different types of highway facilities for 
which different friction factors are assumed herein: (1) rural highways and high-speed urban 
streets, (2) low-speed urban streets, and (3) turning roadways. 



135 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



0.22 

0.20 

£ 0,18 

I 

£ 0.16 
I 0.14 

0.10 
0,08 



._.^ 


...^HRt 


31940M< 


3yer & Be 


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^ Meyer 1949 

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1 

HRB1940 


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20 30 40 60 60 70 80 90 100 110 120 

US CUSf O^AHY 



0.22 

0.20 

£0.18 
B 

I 0.16 

3 0.12 
0.10 
0,08 



10 



R8 1940 Mdyer & Berry 



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layer 1949 






30 



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A 



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40 50 



60 



70 



80 



Exhibit 340. Side Friction Factors 



136 



Elements of Design 



METRIC 



0.5 



04 



c 
o 



0.2 
w 



0.1 




Assumed f<^ c^gn • iaw-^?eed urban streets 




^Assumed lor design - rural "pt 
and high-speed urban highways^ 



10 20 SO 40 60 60 70 SO m 1CK) 110 120 130 

Speed (krr^) 



0.5 



0.4 



I ^^ 




02 



m 



0.1 



\ 



SrrK>otli tfms - w@t concrete pa^^rr^it 



Assyrmd for ensign - Iow*$pe0d urbart streets 




^ Assurmd for design - rural 
and high-speed ytt>an streali 



10 20 



30 40 SO 

Spe@d (flf^) 



60 70 80 



Exhibit 3-lL Comparisoo of Side Friction Faetors Assumed for Desige of Different 

Types of Facilities 



137 



AASHTO — Geometric Design of Highways and Streets 



Distribution of e and f Over a Range of Curves 

For a given design speed there are five methods for sustaining centripetal acceleration on 
curves by use of e or f, or both. These methods are discussed below, and the resulting 
relationships are illustrated in Exhibit 3-12: 

® Method 1 — Superelevation and side friction are directly proportional to the inverse of 
the radius (i.e., a straight-line relation exists between 1/R = and 1/R = 1/Rmin)- 

® Method 2 — Side friction is such that a vehicle traveling at design speed has all lateral 
acceleration sustained by side friction on curves up to those requiring fmax- For sharper 
curves, f remains equal to fmax and superelevation is then used to sustain lateral 
acceleration until e reaches emax* ^^ this method, first f and then e are increased in 
inverse proportion to the radius of curvature. 

® Method 3 — Superelevation is such that a vehicle traveling at the design speed has all 
lateral acceleration sustained by superelevation on curves up to that requiring Cniax- For 
sharper curves, e remains at Cjnax and side friction is then used to sustain lateral 
acceleration until f reaches fmax- In this method, first e and then f are increased in 
inverse proportion to the radius of curvature. 

® Method 4 — This method is the same as method 3, except that it is based on average 
running speed instead of design speed. 

® Method 5 — Superelevation and side friction are in a curvilinear relation with the inverse 
of the radius of the curve, with values between those of methods 1 and 3. 

Exhibit 3-12A compares the relationship between superelevation and the inverse of the 
radius of the curve for these five methods. Exhibit 3-12B shows the corresponding value of side 
friction for a vehicle traveling at design speed, and Exhibit 3-1 2C for a vehicle traveling at the 
corresponding average running speed. 

The straight-line relationship between superelevation and the inverse of the radius of the 
curve in method 1 results in a similar relationship between side friction and the radius for vehicles 
traveling at either the design or average running speed. This method has considerable merit and 
logic in addition to its simplicity. On any particular highway, the horizontal alignment consists of 
tangents and curves of varying radius greater than or equal to the minimum radius appropriate for 
the design speed (Rmin)- Application of superelevation in amounts directly proportional to the 
inverse of the radius would, for vehicles traveling at uniform speed, result in side friction factors 
with a straight-line variation from zero on tangents (ignoring cross slope) to the maximum side 
friction at the minimum radius. This method might appear to be an ideal means of distributing the 
side friction factor, but its appropriateness depends on travel at a constant speed by each vehicle 
in the traffic stream, regardless of whether travel is on a tangent, a curve of intermediate degree, 
or a curve with the minimum radius for that design speed. While uniform speed is the aim of most 
drivers, and can be obtained on well-designed highways when volumes are not heavy, there is a 
tendency for some drivers to travel faster on tangents and the flatter curves than on the sharper 
curves, particularly after being delayed by inability to pass slower moving vehicles. This 
tendency points to the desirability of providing superelevation rates for intermediate curves in 
excess of those that result from use of method 1 . 



758 



Elements of Design 




Corresponding f 
at design speed 



**- 

c 
o 

o 

c 
o 

u 

<u 
-u 
(75 

Ci 


Maximum 




^i-^ '^ 


^-^"^"^^^^^ (4Y Corresp 
^.^'^ at runn 


ondihg f 
ing speed 






1/R 

-c- 





KEY: (^Method of distributing e ond f, refer to text for explanation. 



Exhibit 3-12, Methods of Distributing Superelevation and Side Friction 



739 



AASHTO — Geometric Design of Highways and Streets 



Method 2 uses side friction to sustain all lateral acceleration up to the curvature 
corresponding to the maximum side friction factor, and this maximum side friction factor is 
available on all sharper curves. In this method, superelevation is introduced only after the 
maximum side friction has been used. Therefore, no superelevation is needed on flatter curves 
that need less than maximum side friction for vehicles traveling at the design speed (see curve 2 
in Exhibit 3-12A). When superelevation is needed, it increases rapidly as curves with maximum 
side friction grov^ sharper. Because this method is completely dependent on available side 
friction, its use is generally limited to locations where travel speed is not uniform, such as on 
urban streets. This method is particularly advantageous on low-speed urban streets where, 
because of various constraints, superelevation frequently cannot be provided. 

In method 3, which was practiced many years ago, superelevation to sustain all lateral 
acceleration for a vehicle traveling at the design speed is provided on all curves up to that needing 
maximum practical superelevation, and this maximum superelevation is provided on all sharper 
curves. Under this method, no side friction is provided on flat curves with less than maximum 
superelevation for vehicles traveling at the design speed, as shown by curve 3 in Exhibit 3-12B, 
and the appropriate side friction increases rapidly as curves with maximum superelevation grow 
sharper. Further, as shown by curve 3 in Exhibit 3-12C, for vehicles traveling at average running 
speed, this superelevation method results in negative friction for curves from very flat radii to 
about the middle of the range of curve radii; beyond this point, as curves become sharper, the side 
friction increases rapidly up to a maximum corresponding to the minimum radius of curvature. 
This marked difference in side friction for different curves is not logical and may result in erratic 
driving, either at the design or average running speed. 

Method 4 is intended to overcome the deficiencies of method 3 by using superelevation at 
speeds lower than the design speed. This method has been widely used with an average running 
speed for which all lateral acceleration is sustained by superelevation of curves flatter than that 
needing the maximum rate of superelevation. This average running speed was an approximation 
that, as presented in Exhibit 3-26, varies from 78 to 100 [80 to 100] percent of design speed. 
Curve 4 in Exhibit 3-12A shows that in using this method the maximum superelevation is reached 
near the middle of the curvature range. Exhibit 3-12C shows that at average running speed no 
side friction is needed up to this curvature, and side friction increases rapidly and in direct 
proportion for sharper curves. This method has the same disadvantages as method 3, but they 
apply to a smaller degree. 

To accommodate overdriving that is likely to occur on flat to intermediate curves, it is 
desirable that the superelevation approximate that obtained by method 4. Overdriving on such 
curves involves very little risk that a driver will lose control of the vehicle because superelevation 
sustains nearly all lateral acceleration at the average running speed, and considerable side friction 
is available for greater speeds. On the other hand, method 1, which avoids use of maximum 
superelevation for a substantial part of the range of curve radii, is also desirable. In method 5, a 
curved Une (curve 5, as shown within the triangular working range between curves 1 and 4 in 
Exhibit 3- 12 A) represents a superelevation and side friction distribution reasonably retaining the 
advantages of both methods 1 and 4. Curve 5 has an unsymmetrical parabolic form and represents 
a practical distribution for superelevation over the range of curvature. 



140 



Elements of Design 



Design Considerations 

Superelevation rates that are applicable over the range of curvature for each design speed 
have been determined for use in highway design. One extreme of this range is the maximum 
superelevation rate established by practical considerations and used to determine the maximum 
curvature for each design speed. The maximum superelevation may be different for different 
highway conditions. At the other extreme, no superelevation is needed for tangent highways or 
highways with extremely long-radius curves. For curvature between these extremes and for a 
given design speed, the superelevation should be chosen in such manner that there is a logical 
relation between the side friction factor and the applied superelevation rate. 



Maximym Superelevation Rates 

The maximum rates of superelevation used on highways are controlled by four factors: 
climate conditions (i.e., frequency and amount of snow and ice); terrain conditions (i.e., flat, 
rolling, or mountainous); type of area (i.e., rural or urban); and frequency of very slow-moving 
vehicles whose operation might be affected by high superelevation rates. Consideration of these 
factors jointly leads to the conclusion that no single maximum superelevation rate is universally 
applicable and that a range of values should be used. However, using only one maximum 
superelevation rate within a region of similar climate and land use is desirable, as such a practice 
promotes design consistency. 

Design consistency relates to the uniformity of the highway alignment and its associated 
design element dimensions. This uniformity allows drivers to improve their perception-reaction 
skills by developing expectancies. Design elements that are not uniform for similar types of 
roadways may be counter to a driver's expectancy and result in an increase in driver workload. 
Logically, there is an inherent relationship between design consistency, driver workload, and 
motorist safety with "consistent" designs being associated with lower workloads and safer 
highways. 

The highest superelevation rate for highways in common use is 10 percent, although 
12 percent is used in some cases. Superelevation rates above 8 percent are only used in areas 
without snow and ice. Although higher superelevation rates offer an advantage to those drivers 
traveling at high speeds, current practice considers that rates in excess of 12 percent are beyond 
practical limits. This practice recognizes the combined effects of construction processes, 
maintenance difficulties, and operation of vehicles at low speeds. 

Thus, a superelevation rate of 12 percent appears to represent a practical maximum value 
where snow and ice do not exist. A superelevation rate of 12 percent may be used on low -volume 
gravel-surfaced roads to facilitate cross drainage; however, superelevation rates of this magnitude 
can cause higher speeds, which are conducive to rutting and displacement of gravel. Generally, 
8 percent is recognized as a reasonable maximum value for superelevation rate. 

Where snow and ice are factors, tests and experience show that a superelevation rate of 
about 8 percent is a logical maximum to minimize slipping across a highway by stopped vehicles 

141 



AASHTO — Geometric Design of Highways and Streets 



or vehicles attempting to start slowly from a stopped position. One series of tests (16) found 
coefficients of friction for ice ranging from 0.050 to 0.200, depending on the condition of the ice 
(i.e., wet, dry, clean, smooth, or rough). Tests on loose or packed snow show coefficients of 
friction ranging from 0.200 to 0.400. Other tests (21) have corroborated these values. The lower 
extreme of this range of coefficients of friction probably occurs only under thin film "quick 
freeze" conditions at a temperature of about -PC [30T] in the presence of water on the 
pavement. Similar low friction values may occur with thin layers of mud on the pavement 
surface, with oil or flushed spots, and with high speeds and a sufficient depth of water on the 
pavement surface to permit hydroplaning. For these reasons some highway agencies have adopted 
a maximum superelevation rate of 8 percent. Such agencies believe that 8 percent represents a 
logical maximum superelevation rate, regardless of snow or ice conditions. Such a limit tends to 
reduce the likelihood that slow drivers will experience negative side friction, which can result in 
excessive steering effort and erratic operation. 

Where traffic congestion or extensive marginal development acts to restrict top speeds, it is 
common practice to utilize a low maximum rate of superelevation, usually 4 to 6 percent. 
Similarly, either a low maximum rate of superelevation or no superelevation is employed within 
important intersection areas or where there is a tendency to drive slowly because of turning and 
crossing movements, warning devices, and signals. In these areas it is difficult to warp crossing 
pavements for drainage without providing negative superelevation for some turning movements. 

In summary, it is recommended that (1) several rates, rather than a single rate, of maximum 
superelevation should be recognized in establishing design controls for highway curves, (2) a rate 
of 12 percent should not be exceeded, (3) a rate of 4 or 6 percent is applicable for urban design in 
areas with little or no constraints, and (4) superelevation may be omitted on low-speed urban 
streets where severe constraints are present. Accordingly, five maximum superelevation rates — 
4, 6, 8, 10, and 12 percent — are used below. For each of these rates the maximum curvature and 
actual superelevation rates for flatter curves are determined. In actual design practice, an agency 
will generally use different superelevation rates within the normal range of rates described above 
for different road systems. 

Mioimyrri Radius 

The minimum radius is a limiting value of curvature for a given design speed and is 
determined from the maximum rate of superelevation and the maximum side friction factor 
selected for design (limiting value of f). Use of sharper curvature for that design speed would call 
for superelevation beyond the limit considered practical or for operation with tire friction and 
lateral acceleration beyond what is considered comfortable by many drivers, or both. Although 
based on a threshold of driver comfort, rather than safety, the minimum radius of curvature is a 
significant value in alignment design. The minimum radius of curvature is also an important 
control value for determination of superelevation rates for flatter curves. 

The minimum radius of curvature, /?^,>i, can be calculated directly from the simplified curve 
formula introduced above in the section on the "Side Friction Factor." This formula can be recast 
to determine Rmm as follows: 



142 



Elements of Design 



Metric | 


US Customary | 


^min 


V 


^min 


V' 


(3-10) 


127(0.0k_ + /_) 


15(0.01e_ +/_) 



Design for Rural Highways, Urban Freeways, and High-Speed Urban Streets 

On rural highways, on urban freeways, and on urban streets where speed is relatively high 
and relatively uniform, horizontal curves are generally superelevated and successive curves are 
generally balanced to provide a smooth-riding transition from one curve to the next. A balanced 
design for a series of curves of varying radii is provided by the appropriate distribution of e and f 
values, as discussed above, to select an appropriate superelevation rate in the range from the 
normal cross slope to maximum superelevation. 

Exhibit 3-13 shows the recommended values of the side friction factor for rural highways, 
urban freeways, and high-speed urban streets as a solid line superimposed on the analysis curves 
from Exhibit 3-10. These recommended side friction factors provide a reasonable margin of 
safety at high speeds and lead to somewhat lower superelevation rates for low design speeds than 
do some of the other curves. The lower superelevation rates at the low speeds provide a greater 
margin of safety to offset the tendency of many motorists to overdrive highways with low design 
speeds. 

For the reasons discussed above, it is recommended that maximum side friction factors for 
design of rural highways, urban freeways, and high-speed urban streets should be those 
represented by the solid line in Exhibit 3-13. These maximum side friction factors vary directly 
with design speed from 0.17 at 30 km/h [20 mph] to 0.14 at 80 km/h [50 mph] and then directly 
with design speed from 0.14 at 80 km/h [50 mph] to 0.08 at 130 km/h [80 mph]. The research 
report Side Friction for Superelevation on Horizontal Curves (22) confirms the appropriateness of 
these design values. 

Based on the maximum allowable side friction factors from Exhibit 3-13, Exhibit 3-14 gives 
the minimum radius for each of the five maximum superelevation rates for design speeds from 
20 to 130 km/h [15 to 80 mph]. 

Method 5, described previously, is recommended for the distribution of e and f for all curves 
with radii greater than the minimum radius of curvature on rural highways, urban freeways, and 
high-speed urban streets. Use of method 5 is discussed in the following text and exhibits. 



143 



AASHTO — Geometric Design of Highways and Streets 



mmmc 



0.22 




0.08 



20 30 40 50 ^ 70 80 90 1CK) 110 120 130 

Speed (kn^) 

US CUSTOMARY ^ 



0.22 




40 50 



Exhibit 3-13. Side Frictioe Factors for Rural Highways and High-Speed Urban Streets 



144 



Elements of Design 



Metric I 


US Customary | 


Design 




Limiting 




Calculated Rounded 1 


Design 




Limiting 




Calculated Rounded | 


Speed 


Maximum Values of 


Total 


Radius 


Radius 


Speed 


Maximum Values of 


Total 


Radius 


Radius 


(km/h) 


e(%) 


f 


(e/100 + 


f) (m) 


(m) 


(mph) 


e(%) 


f 


(e/100 + f) 


(ft) 


(ft) 


20 


4.0 


0.18 


0.22 


14.3 


15 


15 


4.0 


0.175 


0.215 


70.0 


70 


30 


4.0 


0.17 


0.21 


33.7 


35 


20 


4.0 


0.170 


0.210 


127.4 


125 


40 


4.0 


0.17 


0.21 


60.0 


60 


25 


4.0 


0.165 


0.205 


203.9 


205 


50 


4.0 


0.16 


0.20 


98.4 


100 


30 


4.0 


0.160 


0.200 


301.0 


300 


60 


4.0 


0.15 


0.19 


149.1 


150 


35 


4.0 


0.155 


0.195 


420.2 


420 


70 


4.0 


0.14 


0.18 


214.2 


215 


40 


4.0 


0.150 


0.190 


563.3 


565 


80 


4.0 


0.14 


0.18 


279.8 


280 


45 


4.0 


0.145 


0.185 


732.2 


730 


90 


4.0 


0.13 


0.17 


375.0 


375 


50 


4.0 


0.140 


0.180 


929.0 


930 


100 


4,0 


0.12 


0.16 


491.9 


490 


55 


4.0 


0.130 


0.170 


1190.2 


1190 














60 


4.0 


0.120 


0.160 


1505.0 


1505 


20 


6.0 


0.18 


0.24 


13.1 


15 


15 


6.0 


0.175 


0.235 


64.0 


65 


30 


6.0 


0.17 


0.23 


30.8 


30 


20 


6.0 


0.170 


0.230 


116.3 


115 


40 


6.0 


0.17 


0.23 


54.7 


55 


25 


6.0 


0.165 


0.225 


185.8 


185 


50 


6.0 


0.16 


0.22 


89.4 


90 


30 


6.0 


0.160 


0.220 


273.6 


275 


60 


6.0 


0.15 


0.21 


134.9 


135 


35 


6.0 


0.155 


0.215 


381.1 


380 


70 


6.0 


0.14 


0.20 


192.8 


195 


40 


6.0 


0.150 


0.210 


509.6 


510 


80 


6.0 


0.14 


0.20 


251.8 


250 


45 


6.0 


0.145 


0.205 


660.7 


660 


90 


6.0 


0.13 


0.19 


335.5 


335 


50 


6.0 


0.140 


0.200 


836.1 


835 


100 


6.0 


0.12 


0.18 


437.2 


435 


55 


6.0 


0.130 


0.190 


1065.0 


1065 


110 


6.0 


0.11 


0.17 


560.2 


560 


60 


6.0 


0.120 


0.180 


1337.8 


1340 


120 


6.0 


0.09 


0.15 


755.5 


755 


65 


6.0 


0.110 


0.170 


1662.4 


1660 


130 


6.0 


0.08 


0.14 


950.0 


950 


70 


6.0 


0.100 


0.160 


2048.5 


2050 














75 


6.0 


0.090 


0.150 


2508.4 


2510 














80 


6.0 


0.080 


0.140 


3057.8 


3060 


20 


8.0 


0.18 


0.26 


12.1 


10 


15 


8.0 


0.175 


0.255 


59.0 


60 


30 


8.0 


0.17 


0.25 


28.3 


30 


20 


8.0 


0.170 


0.250 


107.0 


105 


40 


8.0 


0.17 


0.25 


50.4 


50 


25 


8.0 


0.165 


0.245 


170.8 


170 


50 


8.0 


0.16 


0.24 


82.0 


80 


30 


8.0 


0.160 


0.240 


250.8 


250 


60 


8.0 


0.15 


0.23 


123.2 


125 


35 


8.0 


0.155 


0.235 


348.7 


350 


70 


8.0 


0.14 


0.22 


175.3 


175 


40 


8.0 


0.150 


0.230 


465.3 


465 


80 


8.0 


0.14 


0.22 


228.9 


230 


45 


8.0 


0.145 


0.225 


602.0 


600 


90 


8.0 


0.13 


0.21 


303.6 


305 


50 


8.0 


0.140 


0.220 


760.1 


760 


100 


8.0 


0.12 


0.20 


393.5 


395 


55 


8.0 


0.130 


0.210 


963.5 


965 


110 


8.0 


0.11 


0.19 


501.2 


500 


60 


8.0 


0.120 


0.200 


1204.0 


1205 


120 


8.0 


0.09 


0.17 


666.6 


665 


65 


8.0 


0.110 


0.190 


1487.4 


1485 


130 


8.0 


0.08 


0.16 


831.3 


830 


70 


8.0 


0.100 


0.180 


1820.9 


1820 














75 


8.0 


0.090 


0.170 


2213.3 


2215 














80 


8.0 


0.080 


0.160 


2675.6 


2675 


20 


10.0 


0.18 


0.28 


11.2 


10 


15 


10.0 


0.175 


0.275 


54.7 


55 


30 


10.0 


0.17 


0.27 


26.2 


25 


20 


10.0 


0.170 


0.270 


99.1 


100 


40 


10.0 


0.17 


0.27 


46.6 


45 


25 


10.0 


0.165 


0.265 


157.8 


160 


50 


10.0 


0.16 


0.26 


75.7 


75 


30 


10.0 


0.160 


0.260 


231.5 


230 


60 


10.0 


0.15 


0.25 


113.3 


115 


35 


10.0 


0.155 


0.255 


321.3 


320 


70 


10.0 


0.14 


0.24 


160.7 


160 


40 


10.0 


0.150 


0.250 


428.1 


430 


80 


10.0 


0.14 


0.24 


209.9 


210 


45 


10.0 


0.145 


0.245 


552.9 


555 


90 


10.0 


0.13 


0.23 


277.2 


275 


50 


10.0 


0.140 


0,240 


696.8 


695 


100 


10.0 


0.12 


0.22 


357.7 


360 


55 


10.0 


0.130 


0.230 


879.7 


880 


110 


10.0 


0.11 


0.21 


453.5 


455 


60 


10.0 


0.120 


0.220 


1094.6 


1095 


120 


10.0 


0.09 


0.19 


596.5 


595 


65 


10.0 


0.110 


0.210 


1345.8 


1345 


130 


10.0 


0.08 


0.18 


738.9 


740 


70 


10.0 


0.100 


0.200 


1638.8 


1640 














75 


10.0 


0.090 


0.190 


1980.3 


1980 














80 


10.0 


0.080 


0.180 


2378.3 


2380 


20 


12.0 


0.18 


0.30 


10.5 


10 


15 


12.0 


0.175 


0.295 


51.0 


50 


30 


12.0 


0.17 


0.29 


24.4 


25 


20 


12.0 


0.170 


0.290 


92.3 


90 


40 


12.0 


0.17 


0.29 


43.4 


45 


25 


12.0 


0.165 


0.285 


146.7 


145 


50 


12.0 


0.16 


0.28 


70.3 


70 


30 


12.0 


0.160 


0.280 


215.0 


215 


60 


12.0 


0.15 


0.27 


104.9 


105 


35 


12.0 


0.155 


0.275 


298.0 


300 


70 


12.0 


0.14 


0.26 


148.3 


150 


40 


12.0 


0.150 


0.270 


396.4 


395 


80 


12.0 


0.14 


0.26 


193.7 


195 


45 


12.0 


0.145 


0.265 


511.1 


510 


90 


12.0 


0.13 


0.25 


255.0 


255 


50 


12.0 


0.140 


0.260 


643.2 


645 


100 


12.0 


0.12 


0.24 


327.9 


330 


55 


12.0 


0.130 


0.250 


809.4 


810 


110 


12.0 


0.11 


0.23 


414.0 


415 


60 


12.0 


0.120 


0.240 


1003.4 


1005 


120 


12.0 


0.09 


0.21 


539.7 


540 


65 


12.0 


0.110 


0.230 


1228.7 


1230 


130 


12.0 


0.08 


0.20 


665.0 


665 


70 


12.0 


0.100 


0.220 


1489.8 


1490 














75 


12.0 


0.090 


0.210 


1791.7 


1790 














80 


12.0 


0.080 


0.200 


2140.5 


2140 


Note: In recognition of safety considerations, use 


of emax = 4.1 


D% Should be limited to urban conditions. 





Exhibit 3-14. Minimum Radius for Design of Rural Highways, Urban Freeways, and 
High-Speed Urban Streets Using Limiting Values of e and f 



145 



AASHTO — Geometric Design of Highways and Streets 



Procedure for Development of Finalized e Distribution 

The side friction factors shown as the solid line on Exhibit 3-13 represent the maximum f 
values selected for design for each speed. When these values are used in conjunction with the 
recommended method 5, they determine the f distribution curves for the various speeds. 
Subtracting these computed f values from the computed value of e/100 -h f at the design speed, 
the finalized e distribution is thus obtained (see Exhibit 3-15). The finalized e distribution curves 
resulting from this approach, based on method 5 and used below, are shown in Exhibits 3-16 to 
3-20. 




Exhibit 3-15o Method 5 Procedure for Development of the Finalized e Distribution 



146 



Elements of Design 



METRfC 




1000 20(K) mm 4(m 
R^lua of ourva (m) 



sooo 



omo 



7000 



ySCySTOMARY 




5000 



100CX) 15000 

Radfius of curve (ft) 



zmm 



25000 



Exhibit 3-16» Desige SEperelevation Rates for Maximem Stiperelevation Rate of 4 Percent 



147 



AASHTO — Geometric Design of Highways and Streets 



METRIC 




1000 20m) 3(KK) 4000 

Radius of curve (m) 



B(m 



&3m 



7CKK} 



us CUSTOMARY 




Radit^ of curve (ft) 



Exhibit 3-17, Design Superelevatioe Rates for Maximum Superelevation Rate of 6 Percent 



148 



Elements of Design 



METBIG 




1000 2mo 



3000 4000 

lladius of ci^v8 (m) 



5000 60(X) 7000 



USGUSTOMiyRY 




m^ 8000 12000 16000 

Radius of Guve (ft) 



20(KK) 24000 



Exhibit 3-18. Design Superelevation Rates for Maximum Superelevation Rate of 8 Percent 



149 



AASHTO — Geometric Design of Highways and Streets 



mmnic 




1000 2000 3000 4000 

Radius deuiv©(m) 



5000 



6000 



7(m 



us GUSTOIIAeY 




4000 mm t2Qm leooo 

BidUis olcuive (ft) 



20CKK) 



24000 



Exhibit 3-19. Design Superelevation Rates for Maximum Superelevation Rate of 10 Percent 



150 



Elements of Design 



METRIC 




t(m 



2(XK) 



ZOm 40CK) 

Radius of curve (m) 



5000 



6000 



7000 



ySCUSTOMABY 




4000 80C^ 120CH) 16000 



2m^ 



240(K3 



Exhibit 3"20o Design Seperelevation Rates for Maximum Supereievation Rate of 12 Percent 



As Exhibit 3-15 illustrates, the f distribution curve at the design speed, using method 5, 
results in an unsymmetrical parabolic curve with legs 1 and 2. These legs correspond to curves 4 
and 3-4, respectively, in Exhibit 3-12B. The terms used in the derivation of the equations used to 
compute the f and finalized e distributions are illustrated in Exhibit 3-15. 



151 



AASHTO — Geometric Design of Highways and Streets 



The e and f distributions for method 5 may be derived using the basic curve formula, 
neglecting the (1 - O.Olef) term, using the following sequence of equations: 



Metric 



US Customary 



0.0k + / = 



0.0079V ^ 
R 



where: 
Vd 

^max 
Tmax 
'»min 
Rpi 



then: 



V = design speed, knn/h; 
e = maximum 
superelevation, percent; 
f = maximum allowable 
side friction factor; 
R = minimum radius, 
meters; 

R = radius at the point of 
intersection, PI, of legs (1) 
and (2) of the f distribution 
parabolic curve (= R at the 
point of intersection of 
0.01 e^iax and (0.01 e-f-f)R); 
running speed, km/h 



0.0079 y; 



^^'" 0.01._ + /, 



and 



R 



PI 



0.0079 y^^ 
0.01 ^_ 



Because (0.01 e + f)D - (0.01 e -f f)R = h, at 
point Rpi the equations reduce to the 
following: 



hpj = 



(o.oie_)yj 



2 \ 



-0.01^„ 



where hpi = PI offset from the 1/R axis. 
Also, 



0.0k + / = 



0.067V^ 
R 



where: 



®max 



ipi = 



Vr = 



V = design speed, mph; 
e = maximum 
superelevation, percent; 
f = maximum allowable side 
friction factor; 
R = minimum radius, feet; 
R = radius at the point of 
intersection, PI, of legs (1) 
and (2) of the f distribution 
parabolic curve (= R at the 
point of intersection of 
O-OIOrnax and (0.01 e + f)R); 
running speed, mph 



then: 



i?„ 



0.067 y; 



0.01 e + f 

max J m 



and 



Rp, - 



0.067 y; 



0.01 e„ 



f 



hp, = 



(o.oie.^)vj 



2\ 



- 0.01 e„ 



where hpi = PI offset from the 1/R axis. 
Also, 



S, = 



5729.58 



(3-11) 



(3-12) 



(3-13) 



Because (0.01 e + f)D - (O.Ole + f)R = h, at ( 3-14 ) 

point Rpi the equations reduce to the 

following: 



(3-15) 



152 



Elements of Design 



Metric 



US Customary 



where Si - slope of leg 1 and 



r. _ Jmax ^PI 

^'~ I 1 



^min ^PI 



where S2 = slope of leg 2. 



The equation for the middle ordinate 
(MO) of an unsymnnetrical vertical curve 
is the following: 



M0 = 



2{L,+L,) 



where: Li = 1/Rpi and L2 = 1/Rmm— 1/Rpi. 
It follows that: 



M0 = - 



J. i_ t s,~s, 



where MO = middle ordinate of the f 
distribution curve, and 



where S^ = slope of leg 1 and 



(3-16) 



S2 = 



J max '^Pl 



5729.58 



' I O 



^min ^PI J 



where S2 = slope of leg 2. 



The equation for the middle ordinate (MO) 
of an unsymmetrical vertical curve is the 
following: 



(3»17) 



M0 = 



A^2W2 ^ij 

2{L, + L,) 



where: Li = 5729.58/Rpi and L2 = 

5729.58(1 /Rmin—1/Rpi)- It follows that: 



(3^18) 



MO- 



5729.58 



R. 



1 1 Y 5, - 5, 



^^i„ Rpi 



.^min ) 



(0.0k + f). 



(0.0k_ +f^jR^^ 



R 



in which R = radius at any point. 



where MO = middle ordinate of the f 
distribution curve, and 



(0.0k + /)„ = (°°"'-^/-)^ 

R 



in which R = radius at any point. 



(3-19) 



755 



AASHTO — Geometric Design of Highways and Streets 



Metric 



US Customary 



Use the general vertical curve equation: 
Y 



^xV 



MO 



y^j 



Use the general vertical curve equation: 
Y 



( 3-20 ) 



^x^' 



MO 



v^y 



with 1/R measured from the vertical axis. 
with 1/R< 1/Rpi, 



with 1/R measured from the vertical axis, 
with 1/R< 1/Rp,, 



/i =M0 



(R ^ 

1\ Dl 



PI 






R 



/, -MO 



(Rp,'^ 5729.58(5, ) 



(3-21) 



v^y 



+ 



R 



where: fi = f distribution at any point 1/R 
< 1/Rpi; and 



0.0k, = (O.Ole + f)^ - f, 



where: 0.01 ei - 0.01 e distribution at any 
point 1/R < 1/Rp|. 



where: fi = f distribution at any point 1/R < 
1/Rpi; and 



O.Ole, = (0.0k + f)^ - /, 



where: 0.01 ei = 0.01 e distribution at any 
point 1/R < 1/Rp|. 



( 3-22 ) 



For1/R>1/Rpi, 



For1/R>1/Rpi, 



( 3-23 ) 



/2 =M0 



1 1 



^ 1 I ^^ 



PI 2 



1 1 



R R 



PI 



f ^ MO 

2 



R R 

min 



1 1 



R R 

min PI 



h + 5729.58 5 
PI ^2^ 



1 1 

R R 

PI 



where: fa = f distribution at any point 1/R 
> 1/Rpi; and 

0.0k, = (0.0k + f)^ - f. 



where: 0.01 02 = 0.01 e distribution at any 
point 1/R >1/Rp|. 



where: iz = f distribution at any point 1/R > 
1/Rpi; and 

0.0k, =:(0.0k + /)^ -/, (3=24) 



where: 0.01 62 = 0.01 e distribution at any 
point 1/R >1/Rp|. 



Exhibit 3-15 is a typical layout illustrating the method 5 procedure for development of the 
finalized e distribution. The figure depicts how the f value is determined for 1/R and then 
subtracted from the value of (e/100 + f) to determine e/100. 



154 



Elements of Design 



An example of the procedure to calculate e for a design speed of 80 km/h [50 mph] and an 
of 10 percent is shown below: 



Example 



Metric 



US Customary 



Determine e given: Vd = 80 km/h 

Bmax = 10 percent 

From Exhibit 3^26: Vr = 70 km/h 
From Exhibit 3-1 4: f =0.14 

(maximum allowable 
side friction factor) 

Using the appropriate equations yields: 

Rmin = 21 0.7. Rp, = 387.1 , and hpi = 0.031 

Si = 11.95 and 82 = 50.23 

Substituting, the middle ordinate becomes 
0.022. 

The e distribution value for any radius is 
found by taking the (0.01 e +f)D value minus 
the fi or f2 value (refer to Exhibit 3-15). Thus, 
the e distribution value for an R = Rpi would 
be (0.01 e -^ f)D = 0.0079(Vd)^/R = 0.131 
minus an fi = 0.053, which results in 0.078. 
This value multiplied by 100 to convert to 
percent corresponds to the e value, which 
can be interpolated for R ~ 386 m at the 
80 km/h design speed in Exhibit 3-24. 



Determine e given: Vp = 50 mph 

©max = 10 percent 

From Exhibit 3-26: Vr = 44 mph 
From Exhibit 3-14: f = 0.14 

(maximum allowable 
side friction factor) 

Using the appropriate equations yields: 

Rmin = 697.9, Rp, = 1297.12, and hp, = 0.029 

Si = 0.0066 and S2 = 0.0293 

Substituting, the middle ordinate becomes 
0.0231 . 

The e distribution value for any radius is 
found by taking the (0.01 e -ff)D value minus 
the fi or f2 value (refer to Exhibit 3-15). Thus, 
the e distribution value for an R = Rpi would 
be (0.01 e + f)D = 0.067(Vd)^/R = 0.129 
minus an fi = 0.052, which results in 0.077. 
This value multiplied by 100 to convert to 
percent corresponds to the e value, which 
can be interpolated for R = 1 ,298 ft at the 
50 mph design speed in Exhibit 3-24. 



Design Superelevation Tables 

Exhibits 3-21 to 3-25 show, in addition to length of runoff or transition discussed later in this 
chapter, values of R and the resulting superelevation for different design speeds for each of five 
values of maximum superelevation rate (i.e., for a full range of common design conditions). The 
minimum radii for each of the five maximum superelevation rates were calculated from the 
simplified curve formula, with the use of f values from Exhibit 3-13. Method 5 was used to 
distribute e and f in calculating the appropriate superelevation rates for the remainder of the range 
of curvature. Under all but extreme weather conditions, vehicles can travel safely at speeds higher 
than the design speed on horizontal curves with the superelevation rates indicated in the tables. 
This is due to the development of a radius/superelevation relationship that uses friction factors 
that are generally considerably less than can be achieved. This is illustrated in Exhibit 3-11, 
which compares the friction factors used in design of various types of highway facilities and the 
maximum side friction factors available on certain wet and dry concrete pavements. 



155 



AASHTO — Geometric Design of Highways and Streets 



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Elements of Design 



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AASHTO — Geometric Design of Highways and Streets 



The term ^'normal cross slope" (NC) designates curves that are so flat that the elimination of 
adverse cross slope is not considered necessary, and thus the normal cross slope sections can be 
used. The term ^'remove cross slope" (RC) designates curves where it is adequate to eliminate the 
adverse cross slope by superelevating the entire roadway at the normal cross slope. 



Sharpest Curve Without Superelevatiori 

The minimum rate of cross slope applicable to the traveled way is determined by drainage 
needs. Consistent with the type of highway and amount of rainfall, snow, and ice, the usually 
accepted minimum values for cross slope range from 1.5 percent for high-type surfaces to 
approximately 2.0 percent for low-type surfaces (for further information, see the section on 
"Cross Slope" in Chapter 4). For discussion purposes, a value of 1.5 percent is used below as a 
single value representative of the cross slope for high-type, uncurbed pavements. Steeper cross 
slopes are generally needed where curbs are used to minimize ponding of water on the outside 
through lane. 

The shape or form of the normal cross slope varies. Some States and many municipalities 
use a curved traveled way cross section for two-lane roadways, usually parabolic in form. Others 
employ a straight-line section for each lane. 

Very flat horizontal curves need no superelevation. Traffic entering a curve to the right has 
some superelevation provided by the normal cross slope. Traffic entering a curve to the left has 
an adverse or negative superelevation resulting from the normal cross slope, but with flat curves 
the side friction needed to sustain the lateral acceleration and counteract the negative 
superelevation is small. However, on successively sharper curves for the same speed, a point is 
reached where the combination of lateral acceleration and negative superelevation overcomes the 
allowable side friction, and a positive slope across the entire roadway is desirable to help sustain 
the lateral acceleration. Thus, an important part of superelevation design policy is a criterion for 
the minimum curvature for which superelevation is needed, or conversely, the maximum 
curvature for which a normal roadway cross section is appropriate. 

Many highway agencies express this criterion as a single radius of curvature applicable to all 
design speeds. When using this method, care should be taken to ensure that the cross slope is 
sufficient to provide surface drainage and reduce the potential for vehicle hydroplaning and wet 
weather skidding, especially on flat, high-speed curves. Some agencies use a different criterion 
for maximum superelevation without curvature for each design speed. The latter method is more 
realistic and conforms to the previously discussed superelevation-speed-curvature relationships. 
The maximum curvature for sections without superelevation for each design speed should be 
determined by setting consistently low values of side friction factor, considering the effect of 
normal cross slope and both directions of travel. The result is an increasing radius for 
successively higher design speeds. 

For an average rate of cross slope of 1.5 percent and the superelevation curves of 
Exhibit 3-19 (emax= 10 percent), the corresponding minimum radius for each design speed is 
shown in the third column of Exhibit 3-26. These are curvatures calling for superelevation equal 

766 



Elements of Design 



to the normal cross slope, and therefore indicate the limit of curvature with normal cross slopes. The 
side friction factors developed because of adverse cross slope at both the design speed and the 
average running speed are shown in the right columns. It is evident from their uniform and low values 
over the range of design speeds that these radii are logical limiting values for sections with normal 
cross slopes. 

For a limited range of horizontal curves sharper than those shown in Exhibit 3-26, a practical 
superelevation adjustment can be obtained by retaining the shape of the normal traveled way cross 
section but rotating it around the edge or centerline of the traveled way. This adjustment makes it 
unnecessary to change the screeds used in constructing rigid pavements, and the construction 
procedures are the same on such curves as on tangent sections except that the side forms should be set 
to the proper difference in elevation. This method of eliminating adverse slope results in a steeper 
slope at the lower edge of traveled way than would otherwise be obtained, which may be desirable for 
drainage. However, traffic operating on the higher side of the traveled way does not receive as much 
benefit as it does when the normal section is changed to a plane section for the full width of the 
roadway. 

On a curve sharp enough to need a superelevation rate in excess of about 2.0 percent, a plane 
slope across the whole traveled way should be used. A transition from the normal to a straight-line 
cross slope is needed. For short lengths of highway needing cross-slope reversals, the difficulties and 
extra costs involved in constructing such transitions may supersede the desirable design refinement. 
This practical limit of 2.0 percent corresponds to curves with radii ranging from 700 m [2,290 ft] for 
50-km/h [30-mph] design speeds to about 3,500 m [11,500 ft] for 110-km/h [70-mph] design speeds. 
For curves between these values and those in Exhibit 3-26, the superelevation adjustment can be 
made by rotation of the normal traveled way cross section or, preferably, by change to a plane slope 
across the whole traveled way. 



Effects of Grades 

On long or fairly steep grades, drivers tend to travel faster in the downgrade than in the upgrade 
direction, hi a refined design this tendency should be recognized, and some adjustment in 
superelevation rates should be made. In the case of a divided highway with each roadway 
independently superelevated, or on a one-way ramp, such an adjustment can be readily made. In the 
simplest practical form, values from Exhibits 3-21 to 3-25 can be used directly by assuming a slightly 
higher design speed for the downgrade and a slightly lower design speed for the upgrade. The 
appropriate variation in design speed depends on the particular conditions, especially the rate and 
length of grade and the magnitude of the curve radius in comparison to other curves on the approach 
highway section. 

It is questionable whether similar adjustments should be made on two-lane and multilane 
undivided roadways. In one respect the two directions of traffic tend to balance each other, and 
adjustment of superelevation is not needed. However, the downgrade speed is the most critical, 
and adjustment for it may be desirable in some cases. Although not common practice, lanes can 
be constructed at different cross slopes in the same direction. More practical would be an 
adjustment for the whole traveled way as determined by the downgrade speed, because the extra 



767 



AASHTO — Geometric Design of Highways and Streets 



cross slope would not significantly affect upgrade travel, with the possible exception of heavy 
trucks on long grades. The desirability of avoiding minor changes in design speed should also be 
considered. In general, it is advisable to follow the conunon practice of not making such 
superelevation adjustments on undivided roadways. 



Metric | 


US Customary | 








Resulting side 








Resulting side 








friction factor, f, 








friction factor, f. 




Average 


Minimum 


with adverse cross 




Average 


Minimum 


with adverse cross 


Design 
speed 


running 
speed 


curve 
radius 


slope 


Design 
speed 


running 
speed 


curve 
radius 


slope 


at design at running 


at design at running 


(km/h) 


(km/li) 


(m) 


speed speed i 


(mph) 


(mph) 


(ft) 


speed speed 


20 


20 


200 


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20 


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40 


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50 


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30 


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60 


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35 


32 


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70 


63 


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40 


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80 


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45 


40 


6880 


0.035 0.031 


90 


77 


3030 


0.036 0.030 


50 


44 


8350 


0.035 0.031 


100 


85 


3700 


0.036 0.030 


55 


48 


9960 


0.035 0.031 


110 


91 


4270 


0.037 0.030 


60 


52 


11720 


0.036 0.030 


120 


98 


4990 


0.038 0.030 


65 


55 


13180 


0.036 0.030 


130 


102 


5450 


0.040 0.030 


70 


58 


14730 


0.037 0.030 










75 


61 


16380 


0.038 0.030 










80 


64 


18130 


0.039 0.030 



Exhibit 3-26, Minimum Curve Radius for Section with Normal Cross Slopes (emax = 10%) 

Transition Design Controls 

General Considerations 

The design of transition sections includes consideration of transitions in the roadway cross 
slope and possible transition curves incorporated in the horizontal alignment. The former 
consideration is referred to as superelevation transition and the latter is referred to as alignment 
transition. Where both transition components are used, they occur together over a common 
section of roadway at the beginning and end of the mainline circular curves. 

The superelevation transition section consists of the superelevation runoff and tangent 
runout sections. The superelevation runoff section consists of the length of roadway needed to 
accomplish a change in outside-lane cross slope from zero (flat) to full superelevation, or vice 
versa. The tangent runout section consists of the length of roadway needed to accomplish a 
change in outside-lane cross slope from the normal cross slope rate to zero (flat), or vice versa. 
For reasons of safety and comfort, the pavement rotation in the superelevation transition section 
should be effected over a length that is sufficient to make such rotation imperceptible to drivers. 
To be pleasing in appearance, the pavement edges should not appear distorted to the driver. 



168 



Elements of Design 



In the alignment transition section, a spiral or compound transition curve may be used to 
introduce the main circular curve in a natural manner (i.e., one that is consistent with the driver's 
steered path). Such transition curvature consists of one or more curves aligned and located to 
provide a gradual change in alignment radius. As a result, an alignment transition introduces the 
lateral acceleration associated v^ith the curve in a gentle manner. While such a gradual change in 
path and lateral acceleration is appealing, there is no definitive evidence that transition curves are 
essential to the safe operation of the roadway and, as a result, they are not used by many agencies. 

When a transition curve is not used, the roadway tangent directly adjoins the main circular 
curve. This type of transition design is referred to below as the "tangent-to-curve" transition. 

Some agencies employ spiral curves and use their length to make the appropriate 
superelevation transition. A spiral curve approximates the natural turning path of a vehicle. One 
agency believes that the length of spiral should be based on a 4-s minimum maneuver time at the 
design speed of the highway. Other agencies do not employ spiral curves but empirically 
designate proportional lengths of tangent and circular curve for the same purpose. In either case, 
as far as can be determined, the length of roadway to effect the superelevation runoff should be 
the same for the same rate of superelevation and radius of curvature. 

Review of current design practice indicates that the length of a superelevation runoff section 
is largely governed by its appearance. Spiral transition curve lengths as determined otherwise 
often are shorter than those determined for general appearance, so that theoretically derived spiral 
lengths are replaced with longer empirically derived runoff lengths. A number of agencies have 
established one or more control runoff lengths within a range of about 30 to 200 m 
[100 to 650 ft], but there is no universally accepted empirical basis for determining runoff length, 
considering all likely traveled way widths. In one widely used empirical expression, the runoff 
length is determined as a function of the slope of the outside edge of the traveled way relative to 
the centerline profile. 



TangenMo-Cyrve Transition 

Minimum length of superelevation riinoff. For appearance and comfort, the length of 
superelevation runoff should be based on a maximum acceptable difference between the 
longitudinal grades of the axis of rotation and the edge of pavement. The axis of rotation is 
generally represented by the alignment centerline for undivided roadways; however, other 
pavement reference lines can be used. These lines and the rationale for their use is discussed 
below in the section on "Methods of Attaining Superelevation." 

Current practice is to limit the grade difference, referred to as the relative gradient, to a 
maximum value of 0.50 percent or a longitudinal slope of 1:200 at 80 km/h [50 mph]. In one 
source (23), this same 1:200 slope is used for a design speed of 80 km/h [50 mph] and higher. 
Where design speeds are less than 80 km/h [50 mph], greater relative slopes are used. To reflect 
the importance of the higher design speed and to harmonize with the flatter curving elements, 
both horizontal and vertical, it appears logical to extrapolate the relative slopes for the higher 
design speeds. 

769 



AASHTO — Geometric Design of Highways and Streets 



The maximum relative gradient is varied with design speed to provide longer runoff lengths 
at higher speeds and shorter lengths at lower speeds. Experience indicates that relative gradients 
of 0.80 and 0.35 percent [0.78 and 0.35 percent] provide acceptable runoff lengths for design 
speeds of 20 and 130 km/h [15 and 80 mph], respectively. 

Interpolation between these values provides the maximum relative gradients shown in 
Exhibit 3-27. The maximum relative gradient between profiles of the edges of two-lane traveled 
ways should be double those given in the exhibit. Runoff lengths determined on this basis are 
directly proportional to the total superelevation, which is the product of the lane width and 
superelevation rate. 

Previous editions of this policy have suggested that runoff lengths should be at least equal to 
the distance traveled in 2.0 s at the design speed. This criterion tended to determine the runoff 
lengths of curves with small superelevation rates, high speed, or both. Experience with the 2.0-s 
criterion indicates that the improvement in appearance is outweighed by a tendency to aggravate 
problems associated with pavement drainage in the transition section. In fact, it is noted that some 
agencies do not use this control. From this evidence, it is concluded that a comfortable and 
aesthetically pleasing runoff design can be attained through the exclusive use of the maximum 
relative gradient criterion. 



Metric | 


US Customary | 




Maximum 


Equivalent 




Maximum 


Equivalent 


Design speed 


relative 


maximum 


Design speed 


relative 


maximum 


(km/h) 


gradient (%) 


relative slope 


(mph) 


gradient (%) 


relative slope 


20 


0.80 


1:125 


15 


0.78 


1:128 


30 


0.75 


1:133 


20 


0.74 


1:135 


40 


0.70 


1:143 


25 


0.70 


1:143 


50 


0.65 


1:154 


30 


0.66 


1:152 


60 


0.60 


1:167 


35 


0.62 


1:161 


70 


0.55 


1:182 


40 


0.58 


1:172 


80 


0.50 


1:200 


45 


0.54 


1:185 


90 


0.47 


1:213 


50 


0.50 


1:200 


100 


0.44 


1:227 


55 


0.47 


1:213 


110 


0.41 


1:244 


60 


0.45 


1:222 


120 


0.38 


1:263 


65 


0.43 


1:233 


130 


0.35 


1:286 


70 


0.40 


1:250 








75 


0.38 


1:263 








80 


0.35 


1:286 



Exhibit 3-27. Maximum Relative Gradients 



770 



Elements of Design 



On the basis of the preceding discussion, the minimum length of runoff should be 
determined as: 



Metric 


US Customary 


L.''-''f'(K) 


L, >"■}'' iK) (3-25) 

A 


where: 

U = minimum length of 

superelevation runoff, m; 
A = maximum relative gradient, 

percent; 
ni = number of lanes rotated; 
bw = adjustment factor for number 

of lanes rotated; 
w ~ width of one traffic lane, m 

(typically 3.6 m); 
ed = design superelevation rate, 

percent 


where: 

U = minimum length of 

superelevation runoff, ft; 
A = maximum relative gradient, 

percent; 
ni = number of lanes rotated; 
bw = adjustment factor for number of 

lanes rotated; 
w = width of one traffic lane, ft 

(typically 12 ft); 
ed = design superelevation rate, 

percent 



Equation (3-25) can be used directly for undivided streets or highways where the cross 
section is rotated about the highway centerline and ni is equal to one-half the number of lanes in 
the cross section. More generally, Equation (3-25) can be used for rotation about any pavement 
reference line provided that the rotated width (wui) has a common superelevation rate and is 
rotated as a plane. 

A strict application of the maximum relative gradient criterion provides runoff lengths for 
four-lane undivided roadways that are double those for two-lane roadways; those for six-lane 
undivided roadways would be tripled. While lengths of this order may be considered desirable, it 
is often not practical to provide such lengths in design. On a purely empirical basis, it is 
recommended that minimum superelevation runoff lengths be adjusted downward to avoid 
excessive lengths for multilane roadways. The recommended adjusted factors are presented in 
Exhibit 3-28. 

The adjustment factors listed in Exhibit 3-28 are directly applicable to undivided streets and 
highways. Development of runoff for divided highways is discussed in more detail in the later 
section on "Axis of Rotation with a Median." The topic of runoff superelevation for turning 
roadway designs at intersections and through interchanges is discussed in Chapters 9 and 10, 
respectively. 

Typical minimum superelevation runoff lengths are presented in Exhibit 3-29. The lengths 
shown represent cases where one or two lanes are rotated about a pavement edge. The former 
case is found on two-lane roadways where the pavement is rotated about the centerline or on 
one-lane interchange ramps where the pavement rotation is about an edge line. The latter case is 
found on multilane undivided roadways where each direction is separately rotated about an edge 
line. 



777 



AASHTO — Geometric Design of Highways and Streets 





METRIC 






US QUBTOmAHY 


Number 
of Lanes 
Rotated, 

Hi 


Adjustment 

Factor^ 

bw^ 


Lengtti Increase 
Relative to One- 
lane Rotated 
(~n|b«) 


Number 

of Lanes 

Rotated, 

rii 


Adjustment 

Factor, 

bw^ 


Length Inorease 
Relative to One- 
lane Rotated 


1 


100 


1.0 


1 


100 


10 


1,5 


0.83 


1.25 


1.6 


0.83 


125 


2 


0.75 


15 


2 


0T5 


15 


26 


OJD 


1.75 


2.5 


0.70 


175 


3 


0.67 


2.0 


3 


0.67 


2.0 


3.5 


OM 


2.25 


3.5 


0.64 


2.25 



One lane rototed 



Two lanes rotated 



Three lanes rotated 



lom i 



m 



k9P^ 




Normal section 



»2 lanes 2 lanes' 
Normal section 




3 lones 13 \anm 
Nornnal section 



iorie^5_*J 



RotfiM 



lane 



Rototed sect'jon 




1 2 iones 2 lones 
rotated 



Rotated section 




3 lones " 3 janes 
rotated 

Rotated section 



Exhibit 3-28. Adjustment Factor for Number of Lanes Rotated 



Elimination of the 2.0-s travel-time criterion discussed above results in shorter runoff lengths 
for smaller superelevation rates and higher speeds. However, even the shortest runoff lengths 
(corresponding to a superelevation rate of 2.0 percent) correspond to travel times of 0.6 s, which 
is sufficient to provide a smooth edge-of -pavement profile. 

For high-type alignments, superelevation runoff lengths longer than those shown in 
Exhibit 3-28 may be desirable. In this case, drainage needs or the desire for smoothness in the 
traveled way edge profiles may call for a small increase in runoff length. 

The superelevation runoff lengths given in Exhibit 3-28 are based on 3.6-m [12-ft] lanes. For 
other lane widths, the appropriate runoff length should vary in proportion to the ratio of the actual 
lane width to 3.6 m [12 ft]. Shorter lengths could be applied for designs with 3.0- and 3.3-m 
[10- and 11 -ft] lanes, but considerations of consistency and practicality suggest that the runoff 
lengths for 3.6-m [12-ft] lanes should be used in all cases. 



172 



Elements of Design 



Minimem length of tangent rimoetc The length of tangent runout is determined by the 
amount of adverse cross slope to be removed and the rate at which it is removed. To effect a 
smooth edge of pavement profile, the rate of removal should equal the relative gradient used to 
define the superelevation runoff length. Based on this rationale, the following equation should be 
used to compute the minimum tangent runout length: 



Metric 


US Costomary 




L^="''L^ (3^26) 


where: 

Lt = minimum length of 

tangent runout, m; 
Onc = normal cross slope rate, 

percent; 
ed = design superelevation 

rate, percent; 
U = minimum length of 

superelevation runoff, m 


where: 

Lt = minimum length of tangent 

runout, ft; 
eNc = normal cross slope rate, 

percent; 
Od = design superelevation rate, 

percent; 
Lr = minimum length of 

superelevation runoff, ft 



The tangent runout lengths determined with Equation (3-26) are listed in Exhibit 3-29. 

Location with respect to end of curve. In the tangent-to-curve design, the location of the 
superelevation runoff length with respect to the point of curvature (PC) must be determined. 
Normal practice is to divide the runoff length between the tangent and curved sections and to 
avoid placing the entire runoff length on either the tangent or the curve. With full superelevation 
attained at the PC, the runoff lies entirely on the approach tangent, where theoretically no 
superelevation is needed. At the other extreme, placement of the runoff entirely on the circular 
curve results in the initial portion of the curve having less than the desired amount of 
superelevation. Both of these extremes tend to be associated with a large peak lateral acceleration. 

Experience indicates that locating a portion of the runoff on the tangent, in advance of the 
PC, is preferable, since this tends to minimize the peak lateral acceleration and the resulting side 
friction demand. The magnitude of side friction demand incurred during travel through the runoff 
can vary with the actual vehicle travel path. Observations indicate that a spiral path results from a 
driver's natural steering behavior during curve entry or exit. This natural spiral usually begins on 
the tangent and ends beyond the beginning of the circular curve. Most evidence indicates that the 
length of this natural spiral ranges from 2- to 4-s travel time; however, its length may also be 
affected by lane width and the presence of other vehicles. 

Based on the preceding discussion, locating a portion of the runoff on the tangent is 
consistent with the natural spiral path adopted by the driver during curve entry. In this manner, 
the gradual introduction of superelevation prior to the curve compensates for the gradual increase 
in lateral acceleration associated with the spiral path. As a result, the peak lateral acceleration 
incurred at the PC should theoretically be about equal to 50 percent of the lateral acceleration 
associated with the circular curve. 



173 



AASHTO— Geometric Design of Highways and Streets 





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774 



Elements of Design 



To achieve this balance in lateral acceleration, most agencies locate a portion of the runoff 
length on the tangent prior to the curve. The proportion of runoff length placed on the tangent 
varies from 0.6 to 0.8 (i.e., 60 to 80 percent) with a large majority of agencies using 0.67 
(i.e., 67 percent). Most agencies consistently use a single value of this proportion for all street and 
highway curves. 

Theoretical considerations confirm the desirability of placing a larger portion of the runoff 
length on the approach tangent rather than on the circular curve. Such considerations are based on 
analysis of the acceleration acting laterally on the vehicle while it travels through the transition 
section. This lateral acceleration can induce a lateral velocity and lane shift that could lead to 
operational problems. Specifically, a lateral velocity in an outward direction (relative to the 
curve) requires a driver to make a corrective steer maneuver that produces a path radius sharper 
than that of the roadway curve. Such a critical radius produces an undesirable increase in peak 
side friction demand. Moreover, a lateral velocity of sufficient magnitude to shift the vehicle into 
an adjacent lane (without corrective steering) is also undesirable for safety reasons. 

Analysis of the aforementioned theoretical considerations has led to the conclusion that an 
appropriate allocation of runoff length between the tangent and the curve can minimize the 
aforementioned operational problems (24). The values obtained from the analysis are listed in 
Exhibit 3-30. If used in design, the values listed in Exhibit 3-30 should minimize lateral 
acceleration and the vehicle's lateral motion. Values smaller than those listed tend to be 
associated with larger outward lateral velocities. Values larger than those listed tend to be 
associated with larger lateral shifts. 



Metric | 


US Customary j 


Design 
speed 
(km/h) 


Portion of runoff located prior to 
tine curve 


Design _ 
speed _ 
(mph) 


Portion of runoff located prior to 
the curve 


No. of lanes rotated 


No. of lanes rotated 


1.0 1.5 2.0-2.5 3.0-3.5 


1.0 1.5 2.0-2.5 3.0-3.5 


20-70 
80-130 


0.80 0.85 0.90 0.90 
0.70 0.75 0.80 0.85 


15-45 
50-80 


0.80 0.85 0.90 0.90 
0.70 0.75 0.80 0.85 



Exhibit 3-30. Runoff Locations that Minimize the Vehicle^s Lateral Motion 

Theoretical considerations indicate that values for the proportion of runoff length on the 
tangent in the range of 0.7 to 0.9 (i.e., 70 to 90 percent) offer the best operating conditions; the 
specific value in this range should be dependent on design speed and rotated width. Experience 
obtained from existing practice indicates that deviation from the values in Exhibit 3-30 by 
10 percent should not lead to measurable operational problems. In this regard, use of a single 
value for the proportion of runoff length on the tangent in the range of 0.6 to 0.9 (60 to 
90 percent) for all speeds and rotated widths is considered acceptable. However, refinement of 
this value, based on the trends shown in Exhibit 3-30, is desirable when conditions allow. 

Location v^ith respect to end of curve. In alignment design with spirals, the superelevation 
runoff is effected over the whole of the transition curve. The length of the superelevation runoff 
should be equal to the spiral length for the tangent-to-spiral (TS) transition at the beginning and 
the spiral-to-curve (SC) transition at the end of the circular curve. The change in cross slope 



175 



AASHTO — Geometric Design of Highways and Streets 



begins by removing the adverse cross slope from the lane or lanes on the outside of the curve on a 
length of tangent just ahead of TS (the tangent runout) (see Exhibit 3-37). Between the TS and 
SC, the spiral curve and the superelevation runoff are coincident and the traveled way is rotated 
to reach the full superelevation at the SC. This arrangement is reversed on leaving the curve. In 
this design, the whole of the circular curve has full superelevation. 

Limiting superelevation rates. Theoretical considerations indicate that, when a vehicle is 
traveling through a tangent-to-curve transition, large superelevation rates are associated with 
large shifts in the vehicle's lateral position. In general, such shifts in lateral position can be 
minimized by the proper location of the superelevation runoff section, as described above. 
However, excessively large lateral shifts must be checked by the driver through steering action. 

In recognition of the potential adverse effect that large shifts in lateral position may have on 
safety, the threshold superelevation rates associated with a lateral shift of 1.0 m [3.0 ft] are 
identified in Exhibit 3-31. These limiting superelevation rates do not apply for speeds of 80 km/h 
[50 mph] or more when combined with superelevation rates of 12 percent or less. 



Metric 


US Customary 


Limiting 


Limiting 


superelevation rate 


superelevation rate 


Design speed (km/h) (%) 


Design speed (mph) (%) 


20 8 


15 8 


30 8 


20 8 


40 10 


25 10 


50 11 


30 11 


60 11 


35 11 


70 12 


40 11 




45 12 



Exhibit 3-31. Limiting Superelevation Rates 

Designs that incorporate superelevation in excess of the limiting rates may be associated 
with excessive lateral shift. Therefore, it is recommended that such superelevation rates be 
avoided. However, if they are used, consideration should be given to increasing the width of the 
traveled way along the curve to reduce the potential for vehicle encroachment into the adjacent 
lane. 



Spiral Curve Transitions 



General, Any motor vehicle follows a transition path as it enters or leaves a circular 
horizontal curve. The steering change and the consequent gain or loss of lateral force cannot be 
effected instantly. For most curves, the average driver can follow a suitable transition path within 
the limits of normal lane width. However, combinations of high speed and sharp curvature lead to 
longer transition paths, which can result in shifts in lateral position and sometimes actual 
encroachment on adjoining lanes. In such instances, incorporation of transition curves between 
the tangent and the sharp circular curve, as well as between circular curves of substantially 
different radii, may be appropriate in order to make it easier for a driver to keep his or her vehicle 
within its own lane. 

176 



Elements of Design 



The principal advantages of transition curves in horizontal alignment are the following: 

1. A properly designed transition curve provides a natural, easy-to-follow path for drivers, 
such that the lateral force increases and decreases gradually as a vehicle enters and 
leaves a circular curve. Transition curves minimize encroachment on adjoining traffic 
lanes and tend to promote uniformity in speed. A spiral transition curve simulates the 
natural turning path of a vehicle. 

2. The transition curve length provides a suitable location for the superelevation runoff. 
The transition from the normal pavement cross slope on the tangent to the fully 
superelevated section on the curve can be accomplished along the length of the transition 
curve in a manner that closely fits the speed-radius relationship for vehicles traversing 
the transition. Where superelevation runoff is introduced without a transition curve, 
usually partly on the curve and partly on the tangent, the driver approaching the curve 
may have to steer opposite to the direction of the approaching curve when on the 
superelevated tangent portion in order to keep the vehicle within its lane. 

3. A spiral transition curve also facilitates the transition in width where the traveled way is 
widened on a circular curve. Use of spiral transitions provides flexibihty in 
accomplishing the widening of sharp curves. 

4. The appearance of the highway or street is enhanced by the application of spiral 
transition curves. The use of spiral transitions avoids noticeable breaks in the alignment 
as perceived by drivers at the beginning and end of circular curves. Exhibit 3-32 
illustrates such breaks, which are made more prominent by the presence of 
superelevation runoff. 



Length of Spiral 

Length of spiraL Generally, the Euler spiral, which is also known as the clothoid, is used in 
the design of spiral transition curves. The radius varies from infinity at the tangent end of the 
spiral to the radius of the circular arc at the end that adjoins that circular arc. By definition, the 
radius of curvature at any point on an Euler spiral varies inversely with the distance measured 
along the spiral. In the case of a spiral transition that connects two circular curves having 
different radii, there is an initial radius rather than an infinite value. 

The following equation, developed in 1909 by Shortt (25) for gradual attainment of lateral 
acceleration on railroad track curves, is the basic expression used by some highway agencies for 
computing minimum length of a spiral transition curve: 



Metric 


US Customary | 


^ _ 0.0214V ^ 
RC 




, _ 3.15V' 

JL/ — 

RC 


(3-27) 


where: 




where: 






L = 
V = 
R = 


minimum length of spiral, m; 
speed, km/h; 
curve radius, m; 


L = 
V = 
R = 


minimum length of spiral, ft; 
speed, mph; 
curve radius, ft; 




C = 


rate of increase of lateral 
acceleration, m/s^ 


C = 


rate of increase of lateral 
acceleration, ft/s^ 





777 



AASHTO — Geometric Design of Highways and Streets 





Exhibit 3"32» Transitioe Spirals (23) 



The factor C is an empirical value representing the comfort and safety levels provided by the 
spiral curve. The value of C = 0.3 m/s^ [1 ft/s^] is generally accepted for railroad operation, but 
values ranging from 0.3 to 0.9 m/s^ [1 to 3 ft/s^] have been used for highways. This equation is 
sometimes modified to take into account the effect of superelevation, which results in much 
shorter spiral curve lengths. Highways do not appear to need as much precision as is obtained 
from computing the length of spiral by this equation or its modified form. A more practical 
control for the length of spiral is that it should equal the length needed for superelevation runoff. 



178 



Elements of Design 



Maximum radius for use of a spiral. A review of guidance on the use of spiral curve 
transitions indicates a general lack of consistency among highway agencies. In general, much of 
this guidance suggests that an upper limit on curve radius can be established such that only radii 
below this maximum are likely to obtain safety and operational benefits from the use of spiral 
transition curves. Such a limiting radius has been established by several agencies based on a 
minimum lateral acceleration rate. Such minimum rates have been found to vary from 0.4 to 
1.3 m/s^ [1.3 to 4.25 ft/s^]. The upper end of this range of rates corresponds to the maximum 
curve radius for which some reduction in crash potential has also been noted. For these reasons, it 
is recommended that the maximum radius for use of a spiral should be based on a minimum 
lateral acceleration rate of 1.3 m/s^ [4.25 ft/s^] (20). These radii are listed in Exhibit 3-33. 

The radii listed in Exhibit 3-33 are intended for use by those highway agencies that desire to 
use spiral curve transitions. Exhibit 3-33 is not intended to define radii that require the use of a 
spiral. 



Metric 


US Customary 


Design speed (km/h) Maximum radius (m) 


Design speed (mph) Maximum radius (ft) 


20 24 


15 114 


30 54 


20 203 


40 95 


25 317 


50 148 


30 456 


60 213 


35 620 


70 290 


40 810 


80 379 


45 1025 


90 480 


50 1265 


100 592 


55 1531 


110 716 


60 1822 


120 852 


65 2138 


130 1000 


70 2479 




75 2846 




80 3238 


Note: The safety benefits of spiral curve transitions are likely to be negligible for larger radii. | 



Exhibit 3"33o Maximum Radius for Use of a Spiral Curve Transition 



Minimum length of spiral. Several agencies define a minimum length of spiral based on 
consideration of driver comfort and shifts in the lateral position of vehicles. Criteria based on 
driver comfort are intended to provide a spiral length that allows for a comfortable increase in 
lateral acceleration as a vehicle enters a curve. The criteria based on lateral shift are intended to 
ensure that a spiral curve is sufficiently long to provide a shift in a vehicle's lateral position 
within its lane that is consistent with that produced by the vehicle's natural spiral path. It is 
recommended that these two criteria be used together to determine the minimum length of spiral. 
Thus, the minimum spiral length can be computed as: 



179 



AASHTO — Geometric Design of Highways and Streets 



Metric 


US Custoinarv | 


Ls, min should be the larger of: 


Ls, min Should be the larger of: 






Ls,:ni„ =V24(p^„)R 




Ls,™„ =V24(p^jR 


( 3-28 ) 




or 




or 






V' 




y3 


( 3-29 ) 




^.„.,n- 00214 




^,™„-3.15 






RC 




RC 




where: 




where: 






Ls.min 


- minimum length of spiral, m; 


Ls.min 


= minimum length of spiral, ft; 




Pmin 


= minimum lateral offset 
between the tangent and 
circular curve (0.20 m); 


pmin 


= minimum lateral offset 
between the tangent and 
circular cun/e (0.66 ft); 




R 


= radius of circular curve, m; 


R 


= radius of circular curve, ft; 




V 


= design speed, km/h; 


V 


= design speed, mph; 




C 


maximum rate of change in 
lateral acceleration 
(1.2m/s') 


C 


= maximum rate of change in 
lateral acceleration 
(4 fVs^) 





A value of 0.20 m [0.66 ft] is recommended for pxnin- This value is consistent with the 
minimum lateral shift that occurs as a result of the natural steering behavior of most drivers. The 
recommended minimum value for C is 1.2 m/s^ [4.0 ft/s^]. The use of lower values will yield 
longer, more "comfortable" spiral curve lengths; however, such lengths would not represent the 
minimum length consistent with driver comfort. 

Maximum length of spiral. International experience indicates that there is a need to limit 
the length of spiral transition curves. Safety problems have been found to occur on spiral curves 
that are long (relative to the length of the circular curve). Such problems occur when the spiral is 
so long as to mislead the driver about the sharpness of . the approaching curve. A conservative 
maximum length of spiral that should minimize the likelihood of such problems can be computed 
as: 



Metric 


US Customary | 


s,mcix 


-Mp^Jr 




4,.^ =P<P.JR 


( 3»30 ) 


where: 




where: 






t-s,max ~ 


maximum length of 
spiral, m; 


l-s,max 


maximum length of 
spiral, ft; 




Pmax — 


maximum lateral offset 
between the tangent 
and circular curve 
(1.0 m); 


Pmax 


maximum lateral offset 
between the tangent 
and circular curve 
(3.3 ft); 




R 


radius of circular 
curve, m 


R 


= radius of circular 
curve, ft 





A value of 1.0 m [3.3 ft] is recommended for pmax- This value is consistent with the 
maximum lateral shift that occurs as a result of the natural steering behavior of most drivers. It 
also provides a reasonable balance between spiral length and curve radius. 



180 



Elements of Design 



Desirable length of spiral. A recent study of the operational effects of spiral curve 
transitions (20) found that spiral length is an important design control. Specifically, the most 
desirable operating conditions were noted when the spiral curve length was approximately equal 
to the length of the natural spiral path adopted by drivers. Differences between these two lengths 
resulted in operational problems associated with large lateral velocities or shifts in lateral position 
at the end of the transition curve. Specifically, a large lateral velocity in an outward direction 
(relative to the curve) requires the driver to make a corrective steering maneuver that results in a 
path radius sharper than the radius of the circular curve. Such a critical radius produces an 
undesirable increase in peak side friction demand. Moreover, lateral velocities of sufficient 
magnitude to shift a vehicle into an adjacent lane (without corrective steering) are also 
undesirable for safety reasons. 

Based on these considerations, desirable lengths of spiral transition curves are shown in 
Exhibit 3-34, These lengths correspond to 2.0 s of travel time at the design speed of the roadway. 
This travel time has been found to be representative of the natural spiral path for most 
drivers (20). 

The spiral lengths listed in Exhibit 3-34 are recommended as desirable values for street and 
highway design. Theoretical considerations suggest that significant deviations from these lengths 
tend to increase the shifts in the lateral position of vehicles within a lane that may precipitate 
encroachment on an adjacent lane or shoulder. The use of longer spiral curve lengths that are less 
than Ls,max is acceptable. However, where such longer spiral curve lengths are used, consideration 
should be given to increasing the width of the traveled way on the curve to minimize the potential 
for encroachments into the adjacent lanes. 



Metric | 


US Customary 


Design speed (km/h) 


Spiral length (m) 


Design speed (mph) Spiral length (ft) 


20 


11 


15 44 


30 


17 


20 59 


40 


22 


25 74 


50 


28 


30 88 


60 


33 


35 103 


70 


39 


40 117 


80 


44 


45 132 


90 


50 


50 147 


100 


56 


55 161 


110 


61 


60 176 


120 


67 


65 191 


130 


72 


70 205 
75 220 
80 235 



Exhibit 3-34. Desirable Length of Spiral Curve Transition 



Spiral curve lengths longer than those shown in Exhibit 3-34 may be needed at turning 
roadway terminals to adequately develop the desired superelevation. Specifically, spirals twice as 
long as those shown in Exhibit 3-34 may be needed in such situations. The resulting shift in 
lateral position may exceed 1.0 m [3.3 ft]; however, such a shift is consistent with driver 
expectancy at a turning roadway terminal and can be accommodated by the additional lane width 
typically provided on such turning roadways. 

181 



AASHTO — Geometric Design of Highways and Streets 



Finally, if the desirable spiral curve length shown in Exhibit 3-34 is less than the minimum 
spiral curve length determined from Equations (3-28) and (3-29), the minimum spiral curve 
length should be used in design. 

Length of seperelevatioe runoff. In transition design with a spiral curve, it is 
recommended that the superelevation runoff be accomplished over the length of spiral. For the 
most part the calculated values for length of spiral and length of runoff do not differ materially. 
However, in view of the empirical nature of both, an adjustment in one to avoid having two 
separate sets of design criteria is desirable. The length of runoff is applicable to all superelevated 
curves, and it is recommended that this value should be used for minimum lengths of spiral. In 
this manner, the length of spiral should be set equal to the length of runoff. The change in cross 
slope begins by introducing a tangent runout section just in advance of the spiral curve. Full 
attainment of superelevation is then accomplished over the length of the spiral. In such a design, 
the whole of the circular curve has full superelevation. 

Limiting siiperelevation rates. One consequence of equating runoff length to spiral length 
is that the resulting relative gradient of the pavement edge may exceed the values listed in 
Exhibit 3-27. However, small increases in gradient have not been found to have an adverse effect 
on comfort or appearance. In this regard, the adjustment factors listed in Exhibit 3-28 effectively 
allow for a 50 percent increase in the maximum relative gradient when three lanes are rotated. 

The superelevation rates that are associated with a maximum relative gradient that is 
50 percent larger than the values in Exhibit 3-27 are listed in Exhibit 3-35. If the superelevation 
rate used in design exceeds the rate listed in this table, the maximum relative gradient will be at 
least 50 percent larger than the maximum relative gradient allowed for a tangent-to-curve design. 
In this situation, special consideration should be given to the transition's appearance and the 
abruptness of its edge-of -pavement profile. 



Metric 


US Customary | 


Design 
speed 


Number of lanes rotated 


Design 
speed 


Number of lanes rotated | 








(km/h) 


1 2 3 


(mph) 


1 2 


3 


20 


3.7 1.9 1.3 


15 


4.3 2.2 


1.5 


30 


5.2 2.6 1.7 


20 


5.5 2.8 


1.9 


40 


6.5 3.2 2.2 


25 


6.5 3.3 


2.2 


50 


7.5 3.8 2.5 


30 


7.3 3.7 


2.5 


60 


8.3 4.2 2.8 


35 


8.0 4.0 


2.7 


70 


8.9 4.5 3.0 


40 


8.5 4.3 


2.9 


80 


9.3 4.6 3.1 


45 


8.9 4.5 


3.0 


90 


9.8 4.9 3.3 


50 


9.2 4.6 


3.1 


100 


10.2 5.1 3.4 


55 


9.5 4.8 


3.2 


110 


10.4 5.2 3.5 


60 


9.9 5.0 


3.3 


120 


10.6 5.3 3.5 


65 


10.3 5.2 


3.4 


130 


10.6 5.3 3.5 


70 


10.3 5.2 


3.5 






75 


10.5 5.3 


3.5 






80 


10.5 5.3 


3.5 


Note: Based on desirable length of spiral cur 


\/e transition from Exhibit 3-34. 





Exhibit 3-35. Superelevation Rates Associated With Large Relative Gradients 



182 



Elements of Design 



Length of tangent rimout. The tangent runout length for a spiral curve transition design is 
based on the same approach used for the tangent-to-curve transition design. Specifically, a 
smooth edge of pavement profile is desired such that a common edge slope gradient is maintained 
throughout the superelevation runout and runoff sections. Based on this rationale, the following 
equation can be used to compute the tangent runout length: 



Metric 


US Customary | 






^d 


(3-31) 


where: 
U 

Ls 

ed 

©NC 


= length of tangent runout, 

m; 

length of spiral, m; 
= design superelevation 

rate, percent; 
= normal cross slope rate, 

percent 


where: 
Lt 

u 

ed 
eNc 


- length of tangent runout, 
ft; 

= length of spiral, ft; 

- design superelevation 
rate, percent; 

= normal cross slope rate, 
percent 





The tangent runout lengths obtained from Equation (3-31) are presented in Exhibit 3-36. The 
lengths in this table tend to be longer than desirable for combinations of low superelevation rate 
and high speed. Such long lengths may present safety problems when there is insufficient profile 
grade to provide adequate pavement surface drainage. Such problems can be avoided when the 
profile grade criteria described in the section on "Minimum Transition Grades" are apphed to the 
spiral curve transition. 



Metric | 


US Customary | 


Design 
speed 
(km/h) 


Tangent runout length (m) 


Design 
speed 
(mph) 




Tangent runout length (ft) 




Superelevation rate 




Superelevation rate 




2 4 6 8 10 


2 


4 6 8 


10 


20 


11 _ _ _ _ 


15 


44 


_ _ _ 


— 


30 


17 8 - - - 


20 


59 


30 - - 


— 


40 


22 11 7 


25 


74 


37 25 


— 


50 


28 14 9 - - 


30 


88 


44 29 


— 


60 


33 17 11 8 


35 


103 


52 34 26 


— 


70 


39 19 13 10 


40 


117 


59 39 29 


— 


80 


44 22 15 11 


45 


132 


66 44 33 


- 


90 


50 25 17 13 10 


50 


147 


74 49 37 


- 


100 


56 28 19 14 11 


55 


161 


81 54 40 


— 


110 


61 31 20 15 12 


60 


176 


88 59 44 


— 


120 


67 33 22 17 13 


65 


191 


96 64 48 


38 


130 


72 36 24 18 14 


70 


205 


103 68 51 


41 






75 


220 


110 73 55 


44 






80 


235 


118 78 59 


47 


Notes: 1. 


Based on 2.0% normal cross slope 


). 








2. 


Superelevation rates above 10% 


and cells 


with "-' 


' coincide with a pavement edge 




grade that exceeds the maximum 


relative gradient 


in Exhibit 3-27 by 50% or more. 




These limits apply to roads where 


one lane 


is rotated; lower limits apply when 


more 




lanes are rotated (see Exhibit 3-28 


)■ 









Exhibit 3-36. Tangent Runout Lengtli for Spiral Curve Transition Design 



183 



AASHTO — Geometric Design of Highways and Streets 



Compoynd Curve Transition 

In general, compound curve transitions are most commonly considered for application to 
low-speed turning roadways at intersections. In contrast, tangent-to-curve or spiral curve 
transition designs are more commonly used on street and highway curves. 

Guidance concerning compound curve transition design for turning roadways is provided in 
Chapters 9 and 10. The guidance in Chapter 9 applies to low-speed turning roadway terminals at 
intersections, while the guidance in Chapter 10 applies to interchange ramp terminals. 



Methods of Attaining Superelevation 

Four methods are used to transition the pavement to a superelevated cross section. These 
methods include: (1) revolving a traveled way with normal cross slopes about the centerline 
profile, (2) revolving a traveled way with normal cross slopes about the inside-edge profile, 

(3) revolving a traveled way with normal cross slopes about the outside-edge profile, and 

(4) revolving a straight cross-slope traveled way about the outside-edge profile. Exhibit 3-37 
illustrates these four methods. The methods of changing cross slope are most conveniently shown 
in the exhibit in terms of straight line relationships, but it is emphasized that the angular breaks 
between the straight-line profiles are to be rounded in the finished design, as shown in the exhibit. 

The profile reference line controls for the roadway's vertical alignment through the 
horizontal curve. Although shown as a horizontal line in Exhibit 3-37, the profile reference line 
may correspond to a tangent, a vertical curve, or a combination of the two. In Exhibit 3-37A, the 
profile reference line corresponds to the centerline profile. In Exhibits 3-37B and 3-37C, the 
profile reference line is represented as a ''theoretical'' centerline profile as it does not coincide 
with the axis of rotation. In Exhibit 3-37D, the profile reference line corresponds to the outside 
edge of traveled way. The cross sections at the bottom of each diagram in Exhibit 3-37 indicate 
the traveled way cross slope condition at the lettered points. 

The first method, as shown in Exhibit 3-37A, revolves the traveled way about the centerline 
profile. This method is the most widely used because the change in elevation of the edge of the 
traveled way is made with less distortion than with the other methods. In this regard, one-half of 
the change in elevation is made at each edge. 

The second method, as shown in Exhibit 3-37B, revolves the traveled way about the inside- 
edge profile. In this case, the inside-edge profile is determined as a line parallel to the profile 
reference line. One-half of the change in elevation is made by raising the actual centerline profile 
with respect to the inside-edge profile and the other half by raising the outside-edge profile an 
equal amount with respect to the actual centerline profile. 

The third method, as shown in Exhibit 3-37C, revolves the traveled way about the outside- 
edge profile. This method is similar to that shown in Exhibit 3-37B except that the elevation 
change is accomplished below the outside-edge profile instead of above the inside-edge profile. 



184 



Elements of Design 



Normal i Tangent j Length of j Full 

^^^ ^^.^ : ,4«, ^ . t ,^ H - , n ,„ , ™_^,,.|«i ___„...^ 

Crown j Runout I Runoff 1 Superelevotton 

I ! i 



I 



j I i j Outside edge of 

\ I I Jr^^'^'^^ i traveled way 

^^^ ! ! .---"^'^ I i 

^ l.^^'^T'"^ i 1 Centerline profile 

^^^^^- ^^ — . ^.-«j=-*r^:n-.j -|.^^ j j 

j ^^.. j ^"---^^ j^ Inside edge of 

I ill traveled way 



j I <Nsj <\j ^ Profile control 

<^^^^r^ r^ >S \ centerline 

it i i r 

A B C D E 

CROWNED 

TRAVELED WAY REVOLVED ABOUT CENTERLINE 



! I I 

i ^ i 

Normal < Tongent j Length of Full 

Crown j (Runout j Runoff j Superelevation 

i i i 

j 11 i 1 Outside edge of 

^"^.^ j j j ^^-^^"^ I traveled way 



! Actual r profile 

! f i /" ^^^1 ^,.,-t— — — 

I yi'^l---'"'"" 1 I Theoretlcol Q. profile 

I j [^ ! ! Inside edge of 

j 1 *^--^ j ^1 traveled way 

^xiv i=iv ^^%^ %v IL ^''o^*'® control 

I I j ! inside edge 

II ill traveled way 
A B C D E 

CROWNED 

TRAVELED WAY REVOLVED ABOUT INSIDE EDGE 

~B- 



Exhibit 3-37. Diagrammatic Profiles Showing Methods of Attaiitiog Superelevation for a 

Curve to the Right 



185 



AASHTO — Geometric Design of Highways and Streets 



Norma! 



Length of 



Crown 



Runoff 




Full 



Syperelevqtion 



Theoreticol ?. profile 



i 



Outside edge of 
troveled way 

Actual t profile 

jnside edge of 
traveled woy 

Profile control 



^ \\ V( outside edge of 

^ u traveled way 



A B C D E 

CROWNED 
TRAVELED WAY REVOLVED ABOUT OUTSIDE EDGE 

"C- 



Normat 



I Tarvgerrt__ 



Crown 



I Runout 

I 



Length qf_ 
Runofl 



Full 



"l 



-h 



Supereievotion 



Outside edge of 



"^v 1 






■^^"^^" 



I 

1 

I 
i 
i 
i 
i 



traveled ^ay 



Inside edge of 

traveled way 

Profile control 



^^ ^ ^ outside edge of 

ri Nj traveled way 



A 8 C D E 

STRAIGHT CROSS SLOPE 
TRAVELED WAY REVOLVED ABOUT OUTSIDE EDGE 

»D- 





NOTE: ANGULAR BREAKS TO BE APPROPRiATELY ROUNDED AS SHOWN. (SEE TEXT) 

Exhibit 3-37. Diagrammatic Profiles Showing Methods of Attaining Superelevation for a 

Curve to the Right (Continued) 



186 



Elements of Design 



The fourth method, as shown in Exhibit 3-37D, revolves the traveled v^ay (having a straight 
cross-slope) about the outside-edge profile. This method is often used for two-lane one-way 
roadways where the axis of rotation coincides with the edge of the traveled way adjacent to the 
highway median. 

The methods for attaining superelevation are nearly the same for all four methods. Cross 
section A at one end of the tangent runout is a normal (or straight) cross-slope section. At cross 
section B, the other end of the tangent runout and the beginning of the superelevation runoff, the 
lane or lanes on the outside of the curve are made horizontal (or level) with the actual centerline 
profile for Exhibits 3-37A, 3-37B, and 3-37C; there is no change in cross slope for 
Exhibit 3-37D. 

At cross section C the traveled way is a plane, superelevated at the normal cross slope rate. 
Between cross sections B and C for Exhibits 3-37A, 3-37B, and 3-37C, the outside lane or lanes 
change from a level condition to one of superelevation at the normal cross slope rate and normal 
cross slope is retained on the inner lanes. There is no change between cross sections B and C for 
Exhibit 3-37D. Between cross sections C and E the pavement section is revolved to the full rate 
of superelevation. The rate of cross slope at an intermediate point (e.g., cross section D) is 
proportional to the distance from cross section C. 

In an overall sense, the method of rotation about the centerline shown in Exhibit 3-37A is 
usually the most adaptable. On the other hand, the method shown in Exhibit 3-37B is preferable 
where the lower edge profile is a major control, as for drainage. With uniform profile conditions, 
its use results in the greatest distortion of the upper edge profile. Where the overall appearance is 
to be emphasized, the methods of Exhibits 3-37C and 3-37D are advantageous in that the 
upper-edge profile — the edge most noticeable to drivers — retains the smoothness of the control 
profile. Thus, the shape and direction of the centerline profile may determine the preferred 
method for attaining superelevation. 

Considering the infinite number of profile arrangements that are possible and in recognition 
of such specific problems as drainage, avoidance of critical grades, aesthetics, and fitting the 
roadway to the adjacent topography, no general recommendation for the adoption of any 
particular axis of rotation can be made. To obtain the most pleasing and functional results, each 
superelevation transition section should be considered individually. In practice, any pavement 
reference line used for the axis of rotation may be best suited for the problem at hand. 



Design of Smooth Profiles for Traveled Way Edges 

In the diagrammatic profiles shown in Exhibit 3-37, the tangent profile control lines result in 
angular breaks at cross sections A, C, and E. For general appearance and safety, these breaks 
should be rounded in final design by insertion of vertical curves. Even when the maximum 
relative gradient is used to define runoff length, the length of vertical curve needed to conform to 
the 0.65 [0.66] percent break at the 50-km/h [30-mph] design speed (see Exhibit 3-27) and 0.38 
[0.38] percent break at the 120-km/h [75 mph] design speed need not be great. Where the traveled 
way is revolved about an edge, these grade breaks are doubled to 1.30 [1.32] percent for the 

187 



AASHTO — Geometric Design of Highways and Streets 



50-km/h [30-mph] design speed and to 0.76 [0.76] percent for the 120-kni/h [75-mph] design 
speed. Greater lengths of vertical curve are obviously needed in these cases. Specific criteria for 
the lengths of vertical curves at the breaks in the diagrammatic profiles have not been established. 
For an approximate guide, however, the minimum vertical curve length in meters [feet] can be 
used as numerically equal to the design speed in kilometers per hour [equal to the design speed in 
miles per hour]. Greater lengths should be used where practical, as the general profile condition 
may determine. 

Several methods are available for the development of smooth-edge profiles in superelevation 
transition sections. One method defines the edge profiles on a straight-line basis, as shown in 
Exhibit 3-37, and then develops the profile details based on inserting parabolic vertical curves at 
each edge break. In such cases, the minimum vertical curve length is often set equal to a travel 
time at the design speed of about 0.7 s. This method is laborious when the edge vertical curves 
are superimposed on a centerline vertical curve. However, it does provide an essential control for 
the designer and should yield uniformity of results. 

A second method uses a graphical approach to define the edge profile. The method 
essentially is one of spline-line development. In this method the centerline or other base profile, 
which usually is computed, is plotted on an appropriate vertical scale. Superelevation control 
points are in the form of the break points shown in Exhibit 3-37. Then by means of a spline, curve 
template, ship curve, or circular curve, smooth-flowing lines are drawn to approximate the 
straight-line controls. The natural bending of the spline nearly always satisfies the need for 
minimum smoothing. Once the edge profiles are drawn in the proper relation to one another, 
elevations can be read at the appropriate intervals (as needed for construction control). 

An important advantage of the graphical or spline-line method is the infinite study 
alternatives it affords the designer. Alternate profile solutions can be developed with a minimum 
expenditure of time. The net result is a design that is well suited to the particular control 
conditions. The engineering design labor needed for this procedure is minimal. These several 
advantages make this method preferable to the other methods of developing profile details for 
runoff sections. 

Divided highways warrant a greater refinement in design and greater attention to appearance 
than do two-lane highways because divided highways usually serve much greater traffic volumes. 
Moreover, the cost of such refinements is insignificant compared with the construction cost of the 
divided highway. Accordingly, there should be greater emphasis on the development of smooth- 
flowing traveled way edge profiles for divided highways. 



Axis of Rotation with a Median 

In the design of divided highways, streets, and parkways, the inclusion of a median in the 
cross section influences the superelevation transition design. This influence stems from the 
several possible locations for the axis of rotation. The most appropriate location for this axis 
depends on the width of the median and its cross section. Common combinations of these factors 
and the appropriate corresponding axis location are described in the following three cases: 



188 



Elements of Design 



Case I — The whole of the traveled way, including the median, is superelevated as a plane 
section. Case I should necessarily be limited to narrow medians and moderate superelevation 
rates to avoid substantial differences in elevation of the extreme edges of the traveled way arising 
from the median tilt. Specifically, Case I should be applied only to medians with widths of 4 m 
[15 ft] or less. Superelevation can be attained using a method similar to that shown in Exhibit 3- 
37A except for the two median edges, which will appear as profiles only slightly removed from 
the centerline. 

Case II — The median is held in a horizontal plane and the two traveled ways are rotated 
separately around the median edges. Case II can be applied to any width of median but is most 
appropriate for medians with widths between 4 and 18 m [15 and 60 ft]. By holding the median 
edges level, the difference in elevation between the extreme traveled way edges can be limited to 
that needed to superelevate the roadway. Superelevation transition design for Case II usually has 
the median-edge profiles as the control. One traveled way is rotated about its lower edge and the 
other about its higher edge. Superelevafion can be attained using any of the methods shown in 
Exhibits 3-37B, 3-37C, and 3-37D, with the profile reference line being the same for both 
traveled ways. 

Case III — The two traveled ways are treated separately for runoff with a resulting variable 
difference in elevations at the median edges. Case III design can be used with wide medians (i.e., 
those having a width of 18 m [60 ft] or more). For this case, the differences in elevation of the 
extreme edges of the traveled way are minimized by a compensating slope across the median. 
With a wide median, it is possible to design the profiles and superelevation transition separately 
for the two roadways. Accordingly, superelevation can be attained by the method otherwise 
considered appropriate (i.e., any of the methods in Exhibit 3-37 can be used). 

Superelevation runoff lengths vary for each of the three cases. For Case I designs, the length 
of runoff should be based on the total rotated width (including the median width). Runoff lengths 
for Case II designs should be the same as those for undivided highways with a similar number of 
lanes. Finally, runoff lengths for Case III designs are based on the needs of the separate one-way 
roadways, as defined by their superelevation rates and rotated widths. 

Superelevation runoff lengths for four- and six-lane undivided highways have been shown in 
Exhibit 3-28 as 1.5 and 2 times, respectively, the lengths for two-lane highways. For Case I 
designs of divided highways the length of runoff should properly be increased in the proportion to 
the total width, including the median. Because Case I applies primarily to narrow medians, the 
added length usually will be insignificant. With medians of the order of 1 to 3 m [3 to 10 ft] wide, 
any increase in runoff length may well be ignored. 

Under Case II conditions with narrow medians in a horizontal plane, the runoff lengths 
should be the same as those for undivided highways as shown in Exhibits 3-21 through 3-25 for 
four-lane highways. This length applies to highways with medians about 4 m [15 ft] or less in 
width. However, with medians about 12 m [40 ft] or more in width, the two-lane values should be 
used for the one-way roadways because the extreme traveled way edges are at least 24 m [80 ft] 
apart and are independent of each other. Values for the one-way roadways of six-lane highways 
when separated by a wide median should be 1.2 times the two-lane values of Exhibits 3-21 

189 



AASHTO — Geometric Design of Highways and Streets 



through 3-25. The one-way traveled ways of highways with medians between 4 and 12 m [15 and 
40 ft] might be designed on the basis of either the suggested two-lane or multilane runoff lengths. 

With Case III cross sections, the median generally will be 12 m [40 ft] or more in width, and 
the two-lane values for length of runoff are applicable for one-way roadways of four-lane divided 
highways. The values for the one-way roadways of six-lane divided highways should be 
somewhat greater. In situations where the median width is less than about 12 m [40 ft], the runoff 
length should be determined in the same manner as for Case 11. 

Divided highways warrant a greater refinement in design and greater attention to appearance 
than two-lane highways because they serve much greater traffic volumes and because the cost of 
such refinements is insignificant compared with the cost of construction. Accordingly, the values 
for length of runoff indicated above should be considered minimums, and the use of yet longer 
values should be considered. Likewise, there should be emphasis on the development of smooth- 
flowing traveled way edge profiles of the type obtained by spline-line design methods. 



Minimum Transitiori Grades 

Two potential pavement surface drainage problems are of concem in the superelevation 
transition section. One problem relates to the potential lack of adequate longitudinal grade. This 
problem generally occurs when the grade axis of rotation is equal to, but opposite in sign to, the 
effective relative gradient. It results in the edge of pavement having negligible longitudinal grade, 
which can lead to poor pavement surface drainage, especially on curbed cross sections. 

The other potential drainage problem relates to inadequate lateral drainage due to negligible 
cross slope during pavement rotation. This problem occurs in the transition section where the 
cross slope of the outside lane varies from an adverse slope at the normal cross slope rate to a 
superelevated slope at the normal cross slope rate. This length of the transition section includes 
the tangent runout section and an equal length of the runoff section. Within this length, the 
pavement cross slope may not be sufficient to adequately drain the pavement laterally. 

Two techniques can be used to alleviate these two potential drainage problems. One 
technique is to provide a minimum profile grade in the transition section. The second technique is 
to provide a minimum edge of pavement grade in the transition section. Both techniques can be 
incorporated in the design by use of the following grade criteria: 

1. Maintain minimum profile grade of 0.5 percent through the transition section, 

2. Maintain minimum edge of pavement grade of 0.2 percent (0.5 percent for curbed 
streets) through the transition section. 

The second grade criterion is equivalent to the following series of equations relating profile 
grade and effective maximum relative gradient: 



790 



Elements of Design 



Metric 


US Customary 


Uncurbed Curbed 


Uncurbed Curbed 


G<-A^ 


'-0.2 G<-A*-0.5 


G<-A*-0.2 G<-A*-0.5 


G>-A^ 


^+0.2 G>-A*+0.5 


G^-A*+0.2 G>-A*+0.5 


G<A* 


-0.2 G<A*-0.5 


GsA*-0.2 G<A*-0.5 


G>A* 


+0.2 G>A*+0.5 


G2A*+0.2 G>A*+0.5 


with, 




with, 




^*_(>^«/)erf 


A^*_K)^. (3-32) 




Lr 


Lr 


where: 




where: 


G = 


profile grade, percent; 


G = profile grade, percent; 


A* = 


effective maximum relative 


A* = effective maximum relative 




gradient, percent; 


gradient, percent; 


Lr = 


length of superelevation 


Lr = length of superelevation 




runoff, m; 


runoff, ft; 


n, = 


number of lanes rotated. 


ni = number of lanes rotated, 




lanes; 


lanes; 


w = 


width of one traffic lane, m 


w = width of one traffic lane, ft 




(typically 3.6 m); 


(typically 12 ft); 


Bd = 


design superelevation rate, 


ed = design superelevation rate, 




percent. 


percent. 



The value of 0.2 in the grade control (G) equation represents the minimum edge of pavement 
grade for uncurbed roadways (expressed as a percentage). If this equation is applied to curbed 
streets, the value 0.2 should be replaced with 0.5. 

To illustrate the combined use of the two grade criteria, consider an uncurbed roadway curve 
having an effective maximum relative gradient of 0.65 percent in the transition section. The first 
criterion would exclude grades between -0.50 and +0.50 percent. The second grade criterion 
would exclude grades in the range of -0.85 to -0.45 percent (via the first two components of the 
equation) and those in the range of 0.45 to 0.85 percent (via the last two components of the 
equation). Given the overlap between the ranges for Controls 1 and 2, the profile grade within the 
transition would have to be outside of the range of -0.85 to +0.85 percent in order to satisfy both 
criteria and provide adequate pavement surface drainage. 



Turning Roadway Design 

Turning roadways can be categorized as interchange ramps, roadways, or intersection curves 
for right-turning vehicles. Loop or diamond configurations for turning roadways are used at 
interchanges and consist of combinations of tangents and curves. At intersections, turning 
roadways have a diamond configuration and consist of curves (often compound curves). Turning 
roadway design does not apply to minimum edge-of-traveled-way design for turns at 
intersections. Here it is a matter of closely fitting compound curves to the inside edge of the 
design vehicle's swept path (as described in Chapter 9). 



191 



AASHTO — Geometric Design of Highways and Streets 



When the design speed of the turning roadway is 70 km/h [45 mph] or less, compound 
curvature can be used to form the entire alignment of the turning roadway. When the design speed 
exceeds 70 km/h [45 mph], the exclusive use of compound curves is often impractical, as it tends 
to need a large amount of right-of-way. Thus, high-speed turning roadways follow the interchange 
ramp design guidelines in Chapter 10 and include a mix of tangents and curves. By this approach, 
the design can be more sensitive to right-of-way impacts as well as to driver comfort and safety. 

For compound curves at intersections, it is preferable that the ratio of the flatter radius to the 
sharper radius not exceed 2:1. This ratio results in a reduction of approximately 10 km/h [6 mph] 
in average running speeds for the two curves. 

For compound curves at interchanges, it is preferable that the ratio of the flatter radius to the 
sharper radius not exceed 1.75:1. However, general observations on ramps having differences in 
radii with a ratio of 2:1 indicate that both operation and appearance are satisfactory. 

Curves that are compounded should not be too short or their effect in enabling a change in 
speed from the tangent or flat curve to the sharp curve is lost. In a series of curves of decreasing 
radii, each curve should be long enough to enable the driver to decelerate at a reasonable rate. At 
intersections, a maximum deceleration rate of 5 km/h/s [3 mph/s] may be used (although 3 km/h/s 
[2 mph/s] is desirable). The desirable rate represents very light braking, because deceleration in 
gear alone generally results in overall rates between 1.5 and 2.5 km/h/s [1 and 1.5 mph/s]. 
Minimum compound curve lengths based on these criteria are presented in Exhibit 3-38. 

The compound curve lengths in Exhibit 3-38 are developed on the premise that travel is in 
the direction of sharper curvature. For the acceleration condition, the 2:1 ratio is not as critical 
and may be exceeded. 



Metric 


US Customary | 




Minimum length of 




Minimum length of 


Radius (m) 


circular arc (m) 


Radius (ft) 


circular arc (ft) 


Acceptable Desirable 


Acceptable Desirable 


30 


12 20 


100 


40 60 


50 


15 20 


150 


50 70 


60 


20 30 


200 


60 90 


75 


25 35 


250 


80 120 


100 


30 45 


300 


100 140 


125 


35 55 


400 


120 180 


1 50 or more 


45 60 


500 or more 


140 200 



Exhibit 3-38, Lengths of Circular Arcs for Different Compound Curve Radii 



Design for Low-Speed Urban Streets 



As previously discussed, the maximum allowable side friction factor for use in the design of 
horizontal curves is the point at which the lateral force causes the driver to experience a feeling of 
discomfort when driving a curve at a particular design speed. Exhibit 3-10 has summarized the 

792 



Elements of Design 



test results of side friction factors developed on curves at these apparent limits of comfort. Use of 
the solid line in Exhibit 3-13 and method 5 w^as recommended for distributing e and f in the 
design of rural highways and high-speed urban streets. Method 2 is recommended for the design 
of horizontal curves on low-speed urban streets where, through conditioning, drivers have 
developed a higher threshold of discomfort. By this method, none of the lateral force is 
counteracted by superelevation so long as the side friction factor is less than the specified 
maximum for the radius of the curve and the design speed. 

For sharper curves, f remains at the maximum and e is used in direct proportion to the 
continued increase in curvature until e reaches en,ax- The recommended design values for f that are 
applicable to low-speed urban streets are shown in Exhibit 3-39 as a solid hne superimposed on 
the analysis curves from Exhibit 3-10. They are based on a tolerable degree of discomfort and 
provide a reasonable margin of safety against skidding under normal driving conditions in the 
urban environment. These values vary with the design speed from 0.32 at 30 km/h [0.30 at 20 
mph] to about 0.165 at 70 km/h [0.165 at 45 mph], with 70 km/h [45 mph] being the upper limit 
for low speed established in the design speed discussion of Chapter 2. 

Although superelevation is advantageous for traffic operations, various factors often 
combine to make its use impractical in many built-up areas. These factors include wide pavement 
areas, the need to meet the grade of adjacent property, surface drainage considerations, and 
frequency of cross streets, alleys and driveways. Therefore, horizontal curves on low-speed 
streets in urban areas are frequently designed without superelevation, sustaining the lateral force 
solely with side friction. On these curves for traffic entering a curve to the left the normal cross 
slope is an adverse or negative superelevation, but with flat curves the resultant friction needed to 
sustain the lateral force, even given the negative superelevation, is small. 

However, on successively sharper curves for the same design speed, the minimum radius or 
sharpest curve without superelevation is reached when the side friction factor developed to 
sustain the lateral force, given the adverse cross slope, reaches the maximum allowable value 
based on driver comfort considerations. The maximum allowable side friction factor based on 
driver comfort also provides an appropriate margin of safety against skidding. For travel on 
sharper curves, superelevation is needed. 

The maximum superelevation rate of zero in Exhibit 3-41 establishes the minimum radius 
for each speed below which superelevation is not provided on local streets in residential and 
commercial areas but should be considered in industrial areas or other streets where operating 
speeds are higher. A maximum superelevation rate of 4.0 or 6.0 percent is commonly used. The 
maximum curvature for a given design speed is defined for low-speed urban streets when both the 
maximum superelevation rate and the maximum allowable side friction factors are utilized. 



193 



AASHTO — Geometric Design of Highways and Streets 



^ETBIC 



0.36 
0,34 
0.32 
0.30 
f 0,28 

I 



0.26 



SO.24 



</) 



0,22 
0.20 
0.18 
0.16 
0.14 
0;12 



^ 


r 






















; "'^^J 


s^ ^. Assumed values for low-speed 




- 


\^^ 


uroan assign 










\ 












\ 


\ 








pHRB 1940 Moy©f& Berry \^ 








' f 


Meyer1949-^^ 


\^ 






- 


V 


\ 


\ "^ 


'•^ 




- 


Arizona - 


->'-■■■': 




.,!SJh> ^*^ 












* ^ »» , ^ .'= 




■u*«.rev;^7;.a 


;-rr. 


i — 


HRB 1936 Bametft — 1 






, 1 ...i— 





20 



30 



40 



50 60 

Speed (km^) 



70 



80 



US CUSTOM AMY 



0.34 
0.32 
0.30 
0^8 
0.28 
0.24 
0,22 
0^0 
0,18 
0.16 
014 
0.12 



^^ 














"^"-^^ 
















\.^ 


^^^As^urm 


d values for low 


■speed 








V 


^ urtan di 










1940 Meyer & 8 


X 










HRB 


e»'yy*x^ 












■"^>^*.,r 


Meyer 1949 "-T''"'^^--^ 










. . '•**, 




A 


^^^^-^..^ 








Afizorta— — """^ 


:.*v:--X^^^^ 














t "■■- 




.^. 


-^^nfTnnn 


^t^^zir*^' — 




HRS19:^8amett "^ 









15 



20 



25 



30 35 

Speed (mpN) 



40 



45 



50 



Exhibit 3-39, Side Frictioii Factors for Low-Speed Urban Streets 



194 



Elements of Design 



Rflaximurri Comfortable Speed on Horizontal Curves 

Exhibit 3-40 and 3-41, for low-speed urban streets, are derived from the simplified curve 
formula: 



Metric 


US Customary 


100 '""" 121 R 


100 ' ^'^^ 15R (3-33) 



Exhibit 3-40 has been prepared by using the recommended values of/ for lov^-speed urban 
streets from Exhibit 3-39 and by varying the rates of superelevation from -5.0 to +6.0 percent. 
This exhibit may be used for determining the maximum speeds for horizontal curves on low- 
speed urban streets. By interpolating, the exhibit may also be used to determine the minimum 
superelevation needed where a curve is to be provided with a radius greater than the minimum, 
but less than the radius for a cross section with normal cross slopes. However, it is desirable to 
provide the maximum superelevation for curves with these intermediate radii as well because of 
the tendency for drivers to overdrive curves with lower design speeds. 



Minimum Superelevation Runoff Length 

The following equation for deriving the minimum superelevation runoff length is based on 
the maximum allowable side friction factor, where C is the rate of change of the side friction 
factor obtained from Exhibit 3-41. 



Metric 


US Cystomary 


c 


r 47.2/V^ 

C (3-34) 


where: 

L = length of superelevation runoff, m; 

f = side friction factor; 

Vd = design speed, km/h; and 

C = rate of change of f , nn/s^. 


where: 

L - length of superelevation runoff, ft; 

f = side friction factor; 

Vd = design speed, mph; and 

C = rate of cliange of f , ft/s3. 



195 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



"^ / 'TT I I \ I wf "rim I I IT 

R=15m 20 25 30 40 W 60 70 = 80 90 ; tOO 110120 130 140 150 




20 



35 



40 4$ 50 

Vehlcte spe^, V© (km/h) 



US CUSTOiyiMlY 



6 
5 
4 
3 

I 

I ^ 

Li 

-2 

-4 

-5 



. / III 

RaSQft 75 100 125 


1 1 1 1 1 1 1 1 ! 1 1 1 11/ 1 / 

150 170 2(X> 225 250 275 300 3^350375400425^47^ 500 $5Cf 600 


■ / / 


1 1 


j~T~r 


'III/ 


1 1 1 1 1 1 


1 1 


/ / 


/ / 


1 


1 j 


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• / / 


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15 



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Vehida spsed, Vp (mph) 



40 



45 



Exhibit 3-40, Relationship of Radius Siiperelevatioe, Cross Slope Rate^ and Design Speed 

for Low-Speed Urban Street Design 



196 



Elements of Design 





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CM 


8 


LO 

CO 


O LO 



f^ 



M 



797 



AASHTO — Geometric Design of Highways and Streets 



For design on low-speed urban streets the value of C varies from 1.25 m/s^ at 20 km/h 
[4.25 ft/s^ at 15 mph] to 1.0 m/s^ at 70 km/h [2.75 ft/s^ at 45 mph]. The minimum lengths of 
superelevation runoff for limiting values of superelevation and side friction are shown in 
Exhibit 3-41. The formula for length of superelevation runoff is based on revolving the traveled 
way about the centerline of the street. On flat grades, revolving the traveled way about the 
centerline may result in low spots on the inner edge of the traveled way. To avoid this condition, 
the traveled way should be revolved about the inside edge and the length of superelevation runoff 
shown on Exhibit 3-41 should be doubled. 



MiriimyiTi Radii and Minimum Lengths of Syperelevatlori Runoff for Limiting 
Values of e and f 

Exhibit 3-41 presents the minimum radii for three rates of superelevation: 0.0, 4.0, and 
6.0 percent. The 6.0 percent rate is considered to be the desirable maximum superelevation for 
low-speed urban street design. In addition, the exhibit contains the minimum lengths of 
superelevation runoff for each superelevation rate. From this table, the minimum desirable 
tangent length between two reversing curves of minimum radii can be calculated. The 
superelevation rate of zero is included in the exhibit because an intervening length of tangent is 
needed between reversing curves even if neither is superelevated. As stated previously, the 
portion of the maximum superelevation provided at the PC and FT of the curves can range 
between 60 and 90 percent. The sum of the superelevation runoff lengths outside the PC or FT of 
the curves is the minimum intervening length of tangent. 



Curvature of Turning Roadways and Curvature at Intersections 

Curvature for through roads and streets has been discussed previously. Curvature of turning 
roadways and curvature at high-speed intersections are special cases discussed in the following 
four sections. 



Mioimum Radius for Turning Speed 

As further discussed in Chapter 9, vehicles turning at intersections designed for minimum- 
radius turns have to operate at low speed, perhaps less than 13 km/h [10 mph]. While it is 
desirable and often practical to design for turning vehicles operating at higher speeds, it is often 
appropriate for safety and economy to use lower turning speeds at most intersections. The speeds 
for which these intersection curves should be designed depend on vehicle speeds on the approach 
highways, the type of intersection, and the volumes of through and turning traffic. Generally, a 
desirable turning speed for design is the average running speed of traffic on the highway 
approaching the turn. Designs at such speeds offer little hindrance to smooth flow of traffic and 
may be justified for some interchange ramps or, at intersections, for certain movements involving 
little or no conflict with pedestrians or other vehicular traffic. 



198 



Elements of Design 



Curves at intersections need not be considered in the same category as curves on the open 
highways because the various warnings provided and the anticipation of more critical conditions 
at an intersection permit the use of less liberal design factors. Drivers generally operate at higher 
speeds in relation to the radii on intersection curves than on open highway curves. This increased 
speed is accomplished by the drivers' acceptance and use of higher side friction factors in 
operating on curves at intersections than the side friction factors accepted and used on the 
high-speed highways. 

Several studies (26, 27, 28) have been conducted to determine lateral vehicle placement and 
distribution of speeds on intersection curves. Results of these studies pertinent to speed-curvature 
relationships are plotted in Exhibit 3-42. In the analyses of these data the 95-percentile speed of 
traffic was assumed to be that closely representing the design speed, which generally corresponds 
to the speed adopted by the faster group of drivers. Side friction factors (taking superelevation 
into account) actually developed by drivers negotiating the curves at the 95-percentile speed are 
indicated for 34 locations in Exhibit 3-42. The dashed line at the upper left shows the side friction 
factors used for design of curves on rural highways and high-speed urban streets (Exhibit 3-13). 
Use of this control limit for high speeds, and a friction factor of about 0.5 that could be developed 
at a low speed as the other limit, gives an average or representative curve through the plottings of 
individual observations — a relation between design (95-percentile) speed and side friction factor 
that is considered appropriate for rural and high-speed urban curve design for at-grade 
intersections. 

With this relation established and with logical assumptions for the superelevation rate that 
can be developed on intersection curves, minimum radii for various design speeds are derived 
from the simplified curve formula (see Equation (3-9) in the preceding section on "Theoretical 
Considerations"). Obviously, different rates of superelevation would produce somewhat different 
radii for a given design speed and side friction factor. For design of intersection curves it is 
desirable to establish a single minimum radius for each design speed. This is done by assuming a 
likely minimum rate of superelevation (a conservative value) that can nearly always be obtained 
for certain radii. If more superelevation than this minimum is actually provided, drivers will 
either be able to drive the curves a little faster or drive them more comfortably because of less 
friction. 

In selecting a minimum rate of superelevation it is recognized that the sharper the curve the 
shorter its length and the less of an opportunity for developing a large rate of superelevation. This 
condition applies particularly to intersections where the turning roadway is often close to the 
intersection proper, where much of its area is adjacent to the through traveled way, and where the 
complete turn is made through a total angle of about 90 degrees. Assuming the more critical 
conditions and considering the lengths likely to be available for developing superelevation on 
curves of various radii, the minimum rate of superelevation for derivation purposes is taken as 
that varying from zero at 15 km/h to 9.0 percent at 70 km/h [zero at 10 mph to 10.0 percent at 
45 mph]. By using these rates and the side friction factors of Exhibit 3-42 in the simplified curve 
formula, the minimum radii for intersection curves for operation at design speed are derived as 
shown in Exhibit 3-43. 



799 



AASHTO — Geometric Design of Highways and Streets 



METRIG 



130 
120 
110 

^ too 

% ^0 
I 80 
I 70 

I 
? 50 

1 ^^ 
^ 30 

20 

10 





\ 






LEGEND: Ei^ symbol represents 


' 


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Passenger vehicles sarr^led at eadi 




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0.1 0.2 0.3 0,4 

SEde frtdlon factor, f 



0.5 



OS 



US CUSTOMARY 




LEOEND: Each syitibol repres^its 
m interseotkKi curve ^udM. 
Passer^r vehk^tes sampted at 0s^ 



aa 0.3 0.4 

Side friction factor, f 



Exhibit 3-42. Relation Between Speed and Side Friction Factor on Curves at Intersections 



200 



Elements of Design 



Metric 1 


Design (turning) speed, 


15 


20 


30 


40 


50 


60 


70 




V (km/h) 


















Side friction factor, f 


0.40 


0.35 


0.28 


0.23 


0.19 


0.17 


0.15 




Assumed minimum 


















superelevation, 


0.00 


0.00 


0.02 


0.04 


0.06 


0.08 


0.09 




e/100 


















Total e/1 00 + f 


0.40 


0.35 


0.30 


0.27 


0.25 


0.25 


0.24 




Calculated minimum 


5 


9 


24 


47 


79 


113 


161 




radius, R (m) 


















Suggested minimum 


















radius curve for design 


7 


10 


25 


50 


80 


115 


160 




(m) 


















Average running speed 
(l<m/h) 


15 


20 


28 


35 


42 


51 


57 




Note: For design speeds 


greater 


than 70 km/h, use 


values for open highway conditions. 








US 


Customary 










Design (turning) speed, 
V (mph) 


10 


15 


20 


25 


30 


35 


40 


45 


Side friction factor, f 


0.38 


0.32 


0.27 


0.23 


0.20 


0.18 


0.16 


0.15 


Assumed minimum 


















superelevation, 


0.00 


0.00 


0.02 


0.04 


0.06 


0.08 


0.09 


0.10 


e/100 


















Total e/1 00 + f 


0.38 


0.32 


0.29 


0.27 


0.26 


0.26 


0.25 


0.25 


Calculated minimum 
radius, R (ft) 


18 


47 


92 


154 


231 


314 


426 


540 


Suggested minimum 


















radius curve for design 


25 


50 


90 


150 


230 


310 


430 


540 


(ft) 


















Average running speed 


10 


14 


18 


22 


26 


30 


34 


36 


(mph) 


















Note: For design speeds 


greater 


than 45 mph, use 


values for open highway conditions. 





Exhibit 3-43. Minimiim Radii for Intersection Curves 



The minimum radii of Exhibit 3-43 are represented by the sohd line at the left in 
Exhibit 3-44. The solid line at the upper right shows the relation between design speed and 
minimum radius for open highway conditions, as derived with e values shown at the upper left. 
The joining of the two lines indicates that open-highway conditions on intersection curves are 
approached when the curvature is sufficiently flat to permit operation between 60 and 80 km/h 
[40 and 50 mph]. Thus, in design of intersection curves for design speeds of above 70 km/h 
[45 mph], open highway conditions should be assumed and the design based on Exhibit 3-21 to 
3-25. The square points in Exhibit 3-44 are the observed 95-percentile speeds from the same 
studies represented in Exhibit 3-42 for the locations where more than 50 vehicles were sampled. 
The plotted line fits these points closely, indicating further that the assumptions made in the 
derivation of minimum radii in Exhibit 3-44 are appropriate and that a group of the higher speed 
drivers will use the design speed assumed. 

In addition to the design speed, the average running speed is also used in consideration of 
certain elements of intersection design. The points indicated by crosses in Exhibit 3-44 are actual 
average speeds observed on the same intersection curves referred to. The long dashed line 



201 



AASHTO — Geometric Design of Highways and Streets 



through these points is assumed to represent the average running speed for intersection curves. At 
the right this curve crosses the short dashed Hne, which indicates the average speed for open 
highways. For a given design speed on the solid (upper) curve in Exhibit 3-44, the assumed 
running speed lies vertically below on the dashed curve. These running speeds are shown in the 
last line of Exhibit 3-43. 



Mrr.Ric 



110 
1(K) 
90 
80 
70 
60 
50 
40 
30 
20 
10 














1 1 1 i 1 

Assumed dsii0i speed for 


1 


^ 




^ 




^ 


^ 




















??» 




^ 
















\ 


•^Assumed design speed (or 
in*0fsection curves 


^ 


^ 


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,..'• 


^, 


..*- 












\ 


N. 








./ 


/ 


^ 


"^ 


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\ 


\ 


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X 




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speed f<K open f^hv»ay cufvos 








^ 




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4 


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forintars®c^ncurvss 


















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10 sp 


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sds 


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70 or more 
















m 

1,,,-, 





2 

1 


4 


6 

1 


8 

1.... 


10 

1,.. 







2S ^ 7$ 100 12S 150 m 200 225 2S0 a7S 300 ^ ^0 375 400 42S 4$0 47^ 500 

Radius - Minimum for tnterssction (m) 



yS CUSTOMARY 




100 200 3<X) 400 SOO 6(K) 700 800 900 1<K)0 1100 1200 13(K) 1400 1500 
Radius - Minimum for intersection (ft) 

Exhibit 3-44. Minimiim Radii for Curves at Intersections 



202 



Elements of Design 



The minimum radii established above should be used for design preferably on the inner edge 
of the traveled way rather than on the middle of the vehicle path or the centerline of the traveled 
way. In all cases, as much superelevation as practical up to the appropriate maximum value 
should be developed. For the suggested radii shown in Exhibit 3-43, a superelevation rate of at 
least 8.0 percent is desirable at all locations, and a rate of 8.0 to 10.0 percent for locations where 
snow or ice is not a factor. On those intersection legs where all traffic comes to a stop, as at stop 
signs, a lesser amount of superelevation is usually appropriate. Also where large trucks will be 
using an intersection, use of superelevation may need to be limited because these larger trucks 
may have trouble negotiating intersection curves with superelevation. This is particularly true 
where trucks cross over from a roadway or ramp sloping in one direction to one sloping the other 
way. Where there are a significant number of large trucks for each design speed, flatter curves 
and less superelevation should be provided. Superelevation for curves at intersections is further 
discussed under that heading in Chapter 9. 



Transitions and Compoood Curves 

Drivers turning at intersections and at interchange ramp terminals naturally follow 
transitional travel paths just as they do at higher speeds on the open highway. If facilities are not 
provided for driving in this natural manner, many drivers may deviate from the intended path and 
develop their own transition, sometimes to the extent of encroaching on other lanes or on the 
shoulder. Natural travel paths can best be provided by the use of transition or spiral curves that 
may be inserted between a tangent and a circular arc or between two circular arcs of different 
radii. Practical designs that follow transitional paths may also be developed by the use of 
compound circular curves. Transitioned roadways have the added advantage of providing a 
practical means for changing from a normal to a superelevated cross section. 



Length of Spiral 

Lengths of spirals for use at intersections are determined in the same manner as they are for 
open highways. On intersection curves, lengths of spirals may be shorter than they are on the 
open highway curves, because drivers accept a more rapid change in direction of travel under 
intersection conditions. In other words, C (the rate of change of lateral acceleration on 
intersection curves) may be higher on intersection curves than on open highway curves, where 
values of C ranging from 0.3 to 1.0 m/s^ [1 to 3 ft/sec^] generally are accepted. Rates for curves at 
intersections are assumed to vary from 0.75 m/s^ [2.5 ft/s^] for a turnout speed of 80 km/h 
[50 mph] to 1.2 m/s^ [4.0 ft/s^] for 30 km/h [20 mph]. With the use of these values in the Shortt 
formula (25), lengths of spirals for intersection curves are developed in Exhibit 3-45. The 
minimum lengths of spirals shown are for minimum-radius curves as governed by the design 
speed. Somewhat lesser spiral lengths are suitable for above-minimum radii. 



203 



AASHTO — Geometric Design of Highways and Streets 








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204 



Elements of Design 



Spirals also may be advantageous between two circular arcs of widely different radii. In this 
case, the length of spiral can be obtained from Exhibit 3-45 by using a radius that is the difference 
in the radii of the two arcs. For example, two curves to be connected by a spiral have radii of 
250 and 80 m [820 and 262 ft]. This difference of 170 m [558 ft] is very close to the minimum 
radius of 160 m [550 ft] in Exhibit 3-45 for which the suggested minimum length is about 60 m 
[200 ft]. 

Compound curves at intersections for which the radius of one curve is more than twice the 
radius of the other should have either a spiral or a circular curve of intermediate radius inserted 
between the two. If, in such instances, the calculated length of spiral is less than 30 m [100 ft], it 
is suggested that a length of at least 30 m [100 ft] be used. 



Compound Circular Curves 

Compound circular curves are advantageous in effecting desirable shapes of turning 
roadways for at-grade intersections and for interchange ramps. Where circular arcs of widely 
different radii are joined, however, the alignment appears abrupt or forced, and the travel paths of 
vehicles need considerable steering effort. 

On compound curves for open highways, it is generally accepted that the ratio of the flatter 
radius to the sharper radius should not exceed 1.5:1. For compound curves at intersections where 
drivers accept more rapid changes in direction and speed, the radius of the flatter arc can be as 
much as 100 percent greater than the radius of the sharper arc, a ratio of 2: 1. The ratio of 2: 1 for 
the sharper curves used at intersections results in approximately the same difference (about 
10 km/h [6 mph]) in average running speeds for the two curves. These curves are compounded as 
for a ratio of 1.5: 1 on the flatter curves used on the open highway. General observations on ramps 
having differences in radii with a ratio of 2:1 indicate that both operation and appearance 
normally are satisfactory. 

Where practical, a smaller difference in radii should be used. A desirable maximum ratio is 
1.75:1. Where the ratio is greater than 2:1, a suitable length of spiral or a circular arc of 
intermediate radius should be inserted between the two curves. In the case of very sharp curves 
designed to acconmmodate minimum turning paths of vehicles, it is not practical to apply this ratio 
control. In this case, compound curves should be developed that fit closely to the path of the 
design vehicle to be accommodated, for which higher ratios may be needed as shown in 
Chapter 9. 

Curves that are compounded should not be too short or their effectiveness in enabling smooth 
transitions from tangent or flat-curve to sharp-curve operation may be lost. In a series of curves of 
decreasing radii, each curve should be long enough to enable the driver to decelerate at a 
reasonable rate, which at intersections is assumed to be not more than 5 km/h/s [3 mph/s], 
although 3 km/h/s [2 mph/s] is desirable. Minimum curve lengths that meet these criteria based 
on the running speeds shown in Exhibit 3-44, are indicated in Exhibit 3-46. They are based on a 
deceleration of 5 km/h/s [3 mph/s], and a desirable minimum deceleration of 3 km/h/s [2 mph/s]. 

205 



AASHTO — Geometric Design of Highways and Streets 



The latter deceleration rate indicates very light braking, because deceleration in gear alone 
generally results in overall rates between 1.5 and 2,5 km/h/s [1 and 1.5 mph/s]. 



Metric | 


US Customary | 


Radius (m) 


Length of 


circular arc (m) 


Radius (ft) 


Length of circular arc (ft) 


Minimum 


Desirable 


Minimum Desirable 


30 


12 


20 


100 


40 60 


50 


15 


20 


150 


50 70 


60 


20 


30 


200 


60 90 


75 


25 


35 


250 


80 120 


100 


30 


45 


300 


100 140 


125 


35 


55 


400 


120 180 


1 50 or more 


45 


60 


500 or more 


140 200 



Exhibit 3-46. Length of Circular Arc for a Compoiiod Intersection Curve When Follovi^ed 
by a Curve of One-Half Radius or Preceded by a Curve of Double Radius 

These design guidelines for compound curves are developed on the premise that travel is in 
the direction of sharper curvature. For the acceleration condition, the 2:1 ratio is not as critical 
and may be exceeded. 

Offtracking 

Offtracking is the characteristic, common to all vehicles, although much more pronounced 
with the larger design vehicles, in which the rear wheels do not follow precisely the same path as 
the front wheels when the vehicle negotiates a horizontal curve or makes a turn. When a vehicle 
traverses a curve without superelevation at low speed, the rear wheels track inside the front 
wheels. When a vehicle traverses a superelevated curve, the rear wheels may track inside the 
front wheels more or less than the amount computed on the above basis. This is because of the 
slip angle assumed by the tires with respect to the direction of travel, which results from the side 
friction developed between the pavement and rolling tires. The relative position of the wheel 
tracks depends on the speed and the amount of friction developed to sustain the lateral force not 
sustained by superelevation or, when traveling slowly, by the friction developed to counteract the 
effect of superelevation not compensated by lateral force. At higher speeds, the rear wheels may 
even track outside the front wheels. 

Derivatioo of Design Values for Widening on Horizontal Curves 



In each case, the amount of offtracking, and therefore the amount of widening needed on 
horizontal curves, depends jointly on the length and other characteristics of the design vehicle and 
the radius of curvature negotiated. Selection of the design vehicle is based on the size and 
frequency of the various vehicle types at the location in question. The amount of widening needed 
increases with the size of the design vehicle (for single-unit vehicles or vehicles with the same 
number of trailers or semitrailers) and decreases with increasing radius of curvature. The width 
elements of the design vehicle used in determining the appropriate roadway widening on curves 
include the track width of the design vehicles that may meet or pass on the curve, U; the lateral 

206 



Elements of Design 



clearance per vehicle, C; the width of front overhang of the vehicle occupying the inner lane or 
lanes, Fa; the width of rear overhang, Fb; and a width allowance for the difficulty of driving on 
curves, Z. 



The track width (U) for a vehicle following a curve or making a turn, also known as the 
swept path width, is the sum of the track width on tangent (u) (2.44 or 2.59 m [8.0 or 8.5 ft] 
depending on the design vehicle) and the amount of offtracking. The offtracking depends on the 
radius of the curve or turn, the number and location of articulation points, and the lengths of the 
wheelbases between axles. The track width on a curve (U) is calculated using the equation: 



Metric | 


US Customary | 


U-- 


-u + R-^R'--^^ 


f/ = 


-^u-^R-^R'-J^L', (3-35) 


where: 




where: 




U = 


track width on curve, m; 


U = 


track width on curve, ft; 


u = 


track width on tangent (out- 


u 


track width on tangent (out- 




to-out of tires), m; 




to-out of tires), ft; 


R = 


radius of curve or turn, m; 


R = 


radius of curve or turn, ft; 




and 




and 


Li = 


wheelbase of design vehicle 


Li = 


wheelbase of design vehicle 




between consecutive axles 




between consecutive axles 




(or sets of tandem axles) 




(or sets of tandem axles) 




and articulation points, m. 




and articulation points, ft. 



This equation can be used for any combination of radius and number and length of 
wheelbases. The radius for open highway curves is the path of the midpoint of the front axle; 
however, for most design purposes on two-lane highways, the radius of the curve at the centerline 
of the highway may be used for simplicity of calculations. For turning roadways, the radius is the 
path of the outer front wheel (31). The wheelbases (Li) used in the calculations include the 
distances between each axle and articulation point on the vehicle. For a single-unit truck only the 
distance between the front axle and the drive wheels is considered. For an articulated vehicle, 
each of the articulation points is used to determine U. For example, a tractor/semitrailer 
combination truck has three Lj values that are considered in determining offtracking: (1) the 
distance from the front axle to the tractor drive axle(s), (2) the distance from the drive axle(s) to 
the fifth wheel pivot, and (3) the distance from the fifth wheel pivot to the rear axle(s). In the 
summation process, some terms may be negative, rather than positive, if the articulation point is 
in front of, rather than behind, the drive axle(s) (29) or if there is a rear-axle overhang. Rear-axle 
overhang is the distance between the rear axle(s) and the pintle hook of a towing vehicle (30) in a 
multi-trailer combination truck. Representative values for the track width of design vehicles are 
shown in Exhibit 3-47 to illustrate the differences in relative widths between groups of design 
vehicles. 

The lateral clearance allowance, C, provides for the clearance between the edge of the 
traveled way and nearest wheel path and for the body clearance between vehicles passing or 
meeting. Lateral clearance per vehicle is assumed to be 0.6, 0.75, and 0.9 m [2.0, 2.5, and 3.0 ft] 
for tangent lane widths, Wn, equal to 6.0, 6.6, and 7.2 m [20, 22, and 24 ft], respectively. 



207 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



aoo 




3 4 

Track \^iath. U (m) 



yscysTowARY 



20«X) 



1500 






1000 



Bm 




Tmck width, um) 



Exhibit 3-47. Track Width for Widening of Traveled Way on Curves 



208 



Elements of Design 



The width of the front overhang (Fa) is the radial distance between the outer edge of the tire 
path of the outer front wheel and the path of the outer front edge of the vehicle body. For curves 
and turning roadways. Fa depends on the radius of the curve, the extent of the front overhang of 
the design vehicle, and the wheelbase of the unit itself. In the case of tractor-trailer combinations, 
only the wheelbase of the tractor unit is used. Exhibit 3-48 illustrates relative overhang width 
values for Fa determined from: 



Metric 


US Customary 


F^=7^'+A(2L + A)-i? 


F^=^R^^A(2L-\-A)-R (3-36) 


where: 

A = front overhang of inner 
lane vehicle, m; 

L = wheelbase of single unit 
or tractor, m. 


where: 

A = front overhang of inner 
lane vehicle, ft; 

L = wheelbase of single unit 
or tractor, ft. 



The width of the rear overhang (Fb) is the radial distance between the outer edge of the tire 
path of the inner rear wheel and the inside edge of the vehicle body. For the passenger car (P) 
design vehicle, the width of the body is 0.3 m [1 ft] greater than the width of out-to-out width of 
the rear wheels, making Fb = 0.15 m [0.5 ft]. In the truck design vehicles, the width of body is the 
same as the width out-to-out of the rear wheels, and Fb = 0. 

The extra width allowance (Z) is an additional radial width of pavement to allow for the 
difficulty of maneuvering on a curve and the variation in driver operation. This additional width 
is an empirical value that varies with the speed of traffic and the radius of the curve. The 
additional width allowance is expressed as: 



Metric 


US Customary 


z=o.i(y/V^) 


Z = v/Vr (3-37) 


where: 
V = 


design speed of the highway, 
km/h. 


where: 
V = 


design speed of the highway, 
mph. 



This expression, used primarily for widening of the traveled way on open highways, is also 
applicable to intersection curves. Exhibit 3-49 illustrates the computed values for Z for speeds 
between 20 and 100 km/h [15 and 60 mph]. For the normal range of curve radii at intersections, Z 
resolves into a nearly constant value of 0.6 m [2 ft] by using the speed-curvature relations in 
Exhibit 3-44 for radii in the range of 15 to 150 m [50 to 500 ft]. This added width, as shown 
diagrammatically in Exhibits 3-50 and 3-53, should be assumed to be evenly distributed over the 
traveled way width to allow for the inaccuracy in steering on curved paths. 



209 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



2000 




0.2 
FfOJit overhang* F^ (m) 



US CUSTOMARY 



m^ 




Fftm cwrt)af>0. F^ (ft) 



Exhibit 3-48. Front Overhang for Wideeing of Traveled Way on Cerves 



210 



Elements of Design 



METRIC 



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Exhibit 3-49e Extra Width Allowance for Difficulty of Drivieg oe Traveled Way 

on Curves 



211 



AASHTO — Geometric Design of Highways and Streets 



Traveled Way Widening on Horizontal Curves 

The traveled way on horizontal curves is sometimes widened to make operating conditions 
on curves comparable to those on tangents. On earlier highways with narrow lanes and sharp 
curves, there was considerable need for widening on curves, even though speeds were generally 
low. On modem highways and streets with 3.6 m [12 ft] lanes and high-type alignment, the need 
for widening has lessened considerably in spite of high speeds, but for some conditions of speed, 
curvature, and width it remains appropriate to widen traveled ways. 

Widening is needed on certain curves for one of the following reasons: (1) the design vehicle 
occupies a greater width because the rear wheels generally track inside front wheels (offtracking) 
in negotiating curves, or (2) drivers experience difficulty in steering their vehicles in the center of 
the lane. The added width occupied by the vehicle as it traverses the curve as compared with the 
width of the traveled way on tangent can be computed by geometry for any combination of radius 
and wheelbase. The effect of variation in lateral placement of the rear wheels with respect to the 
front wheels and the resultant difficulty of steering should be accommodated by widening on 
curves, but the appropriate amount of widening cannot be determined as positively as that for 
simple offtracking. 

The amount of widening of the traveled way on a horizontal curve is the difference between 
the width needed on the curve and the width used on a tangent: 



Metric 


US Customary 


^^w^-w^ 




w=W^-W^ 


( 3-38 ) 


where: 




where: 






w = 


widening of traveled way on 
curve, nn; 


w = 


widening of traveled way on 
curve, ft; 




Wo - 


width of traveled way on 
curve, m; 


Wc = 


width of traveled way on 
curve, ft; 




Wn = 


width of traveled way on 
tangent, m 


Wn = 


width of traveled way on 
tangent, ft 





The traveled way width needed on a curve (Wc) has several components related to operation 
on curves, including: the track width of each vehicle meeting or passing, U; the lateral clearance 
for each vehicle, C; width of front overhang of the vehicle occupying the inner lane or lanes. Fa; 
and a width allowance for the difficulty of driving on curves, Z. The application of these 
components is illustrated in Exhibit 3-50. Each of these components is derived in the section on 
^'Derivation of Design Values for Widening on Horizontal Curves," earlier in this chapter. 

To determine width W^, it is necessary to select an appropriate design vehicle. The design 
vehicle should usually be a truck because offtracking is much greater for trucks than for 
passenger cars. The WB-15 [WB~50] design vehicle is considered representative for two-lane 
open-highway conditions. Other design vehicles may be selected however, when representative of 
the actual traffic on a particular facihty. 



212 



Elements of Design 



WB-15 [WB-50] 
Design Vehicle 




Exhibit 3-50. Widening Components on Open Highway Curves (Two-Lane Highways, 

One-Way or Two- Way) 



213 



AASHTO— Geometric Design of Highways and Streets 



The width Wc is calculated by the equation: 



Metric 


US Customary | 


W,: 


= N(U + C)+(N-1)F^^Z 


w. 


= iV(f/ + Cj + (iV»ijF^-hZ (3-39) 


where: 




where: 




N ^ 
U = 

C = 
FA = 

Z = 


= number of lanes; 

= track width of design vehicle 

(out-to-out tires), m; 
= lateral clearance, m; 
= width of front overhang of 

inner-lane vehicle, m; 
= extra width allowance, m 


N 
U 

C 
FA 

Z 


= number of lanes; 

= track width of design vehicle 

(out-to-out tires), ft; 
= lateral clearance, ft; 
= width of front overhang of 

inner-lane vehicle, ft; 
= extra width allowance, ft 



The traveled way widening values for the assumed design condition for a WB-15 [WB-50] 
vehicle on a two-lane highway are presented in Exhibit 3-51. The differences in track widths of 
the SU, WB-12, WB-19, WB-20, WB-20D, WB-30T, and WB-33D [SU, WB-40, WB-62, 
WB-65, WB-67D, WB-IOOT, and WB-109D] design trucks are substantial for the sharp curves 
associated with intersections, but for open highways on which radii are usually larger than 200 m 
[650 ft], with design speeds over 60 km/h [30 mph], the differences are insignificant (see 
Exhibit 3-47). Where both sharper curves (as for a 50 km/h [30 mph] design speed) and large 
truck combinations are prevalent, the derived widening values for the WB-15 [WB-50] truck 
should be adjusted in accordance with Exhibit 3-52. The suggested increases of the tabular values 
for two ranges of radius of curvature are general and will not necessarily result in a full lateral 
clearance C or an extra width allowance Z, as shown in Exhibit 3-49 for the shorter radii. With 
the lower speeds and volumes on roads with such curvature, however, slightly smaller clearances 
may be tolerable. 



Design Values for Traveled Way Widening 

Widening is costly and very little is actually gained from a small amount of widening. It is 
suggested that a minimum widening of 0.6 m [2.0 ft] be used and that lower values in 
Exhibit 3-51 be disregarded. Note that the values in Exhibit 3-51 are for a WB-15 [WB-50] 
design vehicle. For other design vehicles, an adjustment from Exhibit 3-52 should be applied. 
Values in Exhibit 3-51 also are applicable to two-lane, one-way traveled ways (i.e., to each 
roadway of a divided highway or street). Studies show that on tangent alignment somewhat 
smaller clearances between vehicles are used in passing vehicles traveling in the same direction 
as compared with meeting vehicles traveling in opposite directions. There is no evidence that 
these smaller clearances are obtained on curved alignment on one-way roads. Moreover, drivers 
are not in position to judge clearances as well when passing vehicles as when meeting opposing 
vehicles on a curved two-way highway. For this reason and because all geometric elements on a 
divided highway are generally well maintained, widening on a two-lane, one-way traveled way of 
a divided highway should be the same as that on a two-lane, two-way highway, as noted in 
Exhibit 3-51. 



214 



Elements of Design 









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277 



AASHTO — Geometric Design of Highways and Streets 



On four-lane undivided highways or streets the widening of the traveled way should be 
double the design values indicated in Exhibit 3-51. This means that some values below 0.6 m 
[2 ft] in Exhibit 3-51, which were disregarded for two-lane highways, may now be used because, 
when doubled for undivided four-lane highways, they will be greater than the minimum. 

The above values are applicable to open-highway curves. For intersection conditions, with 
generally smaller radii on turning roadways, the criteria for design widths are somewhat different. 
These criteria are presented in the section "Widths for Turning Roadways at Intersections" in this 
chapter, and design values are given in Exhibit 3-53. 



Application of Widening on Curves 

Widening should transition gradually on the approaches to the curve to ensure a reasonably 
smooth alignment of the edge of the traveled way and to fit the paths of vehicles entering or 
leaving the curve. The principal points of concern in the design of curve widening, which apply to 
both ends of highway curves, are presented below: 

® On simple (unspiraled) curves, widening should be applied on the inside edge of the 
traveled way only. On curves designed with spirals, widening may be applied on the 
inside edge or divided equally on either side of the centerline. In the latter method, 
extension of the outer-edge tangent avoids a slight reverse curve on the outer edge. In 
either case, the final marked centerline, and desirably any central longitudinal joint, 
should be placed midway between the edges of the widened traveled way. 

® Curve widening should transition gradually over a length sufficient to make the whole 
of the traveled way fully usable. Although a long transition is desirable for traffic 
operation, it may result in narrow pavement slivers that are difficult and expensive to 
construct. Preferably, widening should transition over the superelevation runoff length, 
but shorter lengths are sometimes used. Changes in width normally should be effected 
over a distance of 30 to 60 m [100 to 200 ft]. 

® From the standpoints of usefulness and appearance, the edge of the traveled way 
through the widening transition should be a smooth, graceful curve. A tangent transition 
edge should be avoided. On minor highways or in cases where plan details are not 
available, a curved transition staked by eye generally is satisfactory and better than a 
tangent transition. In any event, the transition ends should avoid an angular break at the 
pavement edge. 

® On highway alignment without spirals, smooth and fitting alignment results from 
attaining widening with one-half to two-thirds of the transition length along the tangent 
and the balance along the curve. This is consistent with a common method for attaining 
superelevation. The inside edge of the traveled way may be designed as a modified 
spiral, with control points determined by the width/length ratio of a triangular wedge, 
by calculated values based on a parabolic or cubic curve, or by a larger radius 
(compound) curve. Otherwise, it may be aligned by eye in the field. On highway 
alignment with spiral curves, the increase in width is usually distributed along the 
length of the spiral. 



275 



Elements of Design 




when C = 1.2 m [4 ft], ond Z = 0.6 m [2 ft] 
then W « U 4. 1.8 [W «^ U -*- 6] 

CASE 1 
ONE-LANE ONE-WAY OPERATION - NO PASSING 




Sinc« possing a stalled vehicl® is ot low speed. Z ^ m [ft]; C/2- 
ond C is assumed half thot for Cos#s i at III. or C » 0,6 m [2 ft] 

then W » U + U + F 4 F^ -f 1.2 [W » u. -f U, 4- f -h F^ -^ 4] 

CASE II 
ONE-LANE ONE-WAY OPERATION PROVISION FOR PASSING STALLED VEHICLE 




[i^S?^:: 




W = U 4 U -h 2C 4- F + F + Z C/2^— ...^^ 

When C ^ 1.2 m [4 ft], and 2 *> 0.6 m [2 ft] 1-^ 

then W « U^ -f U 4 F 4 F^ -^ 5 [W =* U + U, 4^ F -^ F, -I- 10] 

CASE Hi 
TWO-LANE OPERATION - ONE OR TWO WAY 

U - Trock width of vehicle (out-to-out tires), m [ft] C ^ Totol lotero) cleorance per vehicle, m [ft] 
F^ - Width of front overhong. nr^ [ft] ^ ^ ^^^^^ ^j^^^ ollowonce due to difficulty 

F^ = Width of rear overhang^ m [ft] of driving on curves, m [ft] 



Exhibit 3-53. Derivation of Tiireing Roadway Widths on Corves at Intersections 



219 



AASHTO — Geometric Design of Highways and Streets 



® Widening areas can be fully detailed on construction plans. Alternatively, general 
controls can be cited on construction or standard plans with final details left to the field 
engineer. 

Widths for Turning Roadways at Intersections 

The widths of turning roadways at intersections are governed by the types of vehicles to be 
accommodated, the radius of curvature, and the expected speed. Turning roadways may be 
designed for one- or two-way operation, depending on the geometric pattern of the intersection. 

Selection of an appropriate design vehicle should be based on the size and frequency of 
vehicle types using or expected to use the facility. The radius of curvature in combination with 
the track width of the design vehicle determine the width of a turning roadway. The width 
elements for the turning vehicle, shown diagrammatically in Exhibit 3-53, are explained in the 
section on "Derivation of Design Values for Widening on Horizontal Curves," presented earlier 
in this chapter. They ignore the effects of insufficient superelevation and of surfaces with low 
friction resistance that tend to cause the rear wheels of vehicles traveling at other than low speed 
to swing outward, developing the appropriate slip angles. 

Turning roadways are classified for operational purposes as one-lane operation, with or 
without opportunity for passing a stalled vehicle, and two-lane operation, either one-way or 
two-way. Three cases are commonly considered in design: 

Case I^One-lane, one-way operation with no provision for passing a stalled vehicle is 
usually appropriate for minor turning movements and moderate turning volumes where the 
connecting roadway is relatively short. Under these conditions, the chance of a vehicle 
breakdown is remote but one of the edges of the traveled way should preferably have a sloping 
curb or be flush with the shoulder. 

Case II — One-lane, one-way operation with provision for passing a stalled vehicle is used to 
allow operation at low speed and with sufficient clearance so that other vehicles can pass a stalled 
vehicle. These widths are applicable to all turning movements of moderate to heavy traffic 
volumes that do not exceed the capacity of a single-lane connection. In the event of a breakdown, 
traffic flow can be maintained at a somewhat reduced speed. Many ramps and connections at 
channelized intersections are in this category. However, for Case II, the widths needed for the 
longer vehicles are very large as shown in Exhibit 3-54. Case I widths for these longer vehicles, 
including the WB-19, WB-20, WB-30T, and WB-33D [WB-62, WB-65, WB-IOOT, and WB- 
109D] design vehicles, may have to be used as the minimum values where they are present in 
sufficient numbers to be considered the appropriate design vehicle. 

Case III — Two-lane operation, either one- or two-way, is applicable where operation is two 
way or where operation is one way, but two lanes are needed to handle the traffic volume. 



220 



Elements of Design 



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Elements of Design 



Design Values 

The total width, W, for separate turning roadways at intersections is derived by the 
summation of the proper width elements. The separate formulas for width and values for lateral 
clearance, C, and the allowance for difficulty of driving on curves, Z, for each case are shown in 
Exhibit 3-53. Values for track width, U, are obtained from Exhibit 3-47 and values for front 
overhang, Fa, from Exhibit 3-48. Values of U and Fa are read from the exhibit for the turning 
radius, Rj, which is closely approximated by adding the track width and proper clearances to the 
radius of the inner edge of the turning roadway. 

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appropriate. The allowance for difficulty of driving curves, Z, is constant, equal to about 0.6 m 
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considered because no passing of another vehicle is involved. 

For Case II, the width involves U and C for the stopped vehicle and the U and C for the 
passing vehicle. To this is added extra width for the front overhang, Fa, of one vehicle and the 
rear overhang, Fb, (if any) of the other vehicle. The width of rear overhang for a passenger car is 
considered to be 0.15 m [0.5 ft]. Fb for truck design vehicles is 0. A total clearance of one-half 
the value of C in the other two cases is assumed (i.e., 0.6 m [2 ft] for the stopped vehicle and 
0.6 m [2 ft] for the passing vehicle). Because passing the stalled vehicle is accomplished at low 
speeds, the extra width allowance, Z, is omitted. 

All the width elements apply for Case III. To the values of U and Fa obtained from 
Exhibits 3-47 and 3-48, respectively, the lateral clearance, C, of 1.2 m [4 ft], Fb of 0.15 m [0.5 ft] 
for passenger cars, and Z of 0.6 m [2 ft] is added to determine the total width. 

The derived widths for various radii for each design vehicle are given in Exhibit 3-54. For 
general design use, the recommended widths given in Exhibit 3-54 seldom apply directly, 
because the turning roadways usually accommodate more than one type of vehicle. Even 
parkways designed primarily for P vehicles are used by buses and maintenance trucks. At the 
other extreme, few if any public highways are designed to fully accommodate the WB-15 
[WB-50] or longer design vehicles. Widths needed for some combination of separate design 
vehicles become the practical design guide for intersection roadways. Such design widths are 
given in Exhibit 3-55 for three logical conditions of mixed traffic, which are defined below. 
However, where the larger design vehicles such as the WB-19 or WB-33D [WB-62 or WB-109D] 
will be using a turning roadway or ramp, the facility should accommodate their turning paths for 
at least the Case I condition. Therefore, Case I widths for the appropriate design vehicle and 
radius shown in Exhibit 3-54 should be checked to determine whether they exceed widths shown 
in Exhibit 3-55. If they do, consideration should be given to using the widths for Case I shown in 
Exhibit 3-54 as the minimum widths for the turning roadway or ramp. 

Traffic conditions for defining turning roadway widths are described in broad terms because 
data concerning the traffic volume, or the percentage of the total volume, for each type of vehicle 
are not available to define these traffic conditions with precision in relation to width. 



223 



AASHTO — Geometric Design of Highways and Streets 



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Elements of Design 



Traffic Coedition A. This traffic condition consists predominantly of P vehicles, but some 
consideration is also given to SU trucks; the values in Exhibit 3-55 are somewhat higher than 
those for P vehicles in Exhibit 3-54. 

Traffic Condition B. This traffic condition includes sufficient SU trucks to govern design, 
but some consideration is also given to tractor-semitrailer combination trucks; values in Exhibit 
3-55 for Cases I and III are those for SU vehicles in Exhibit 3-54. For Case II, values are reduced 
as explained later in this section. 

Traffic Condition C. This traffic condition includes sufficient tractor-semitrailer 
combination trucks, WB-12 or WB-15 [WB-40 or WB-50], to govern design; the values in 
Exhibit 3-55 for Cases I and III are those for the WB-12 [WB'40] truck in Exhibit 3-54. For 
Case II, values are reduced. 

In general, Traffic Condition A may be assumed to have a small volume of trucks or only an 
occasional large truck; Traffic Condition B, a moderate volume of trucks (e.g., in the range of 5 
to 10 percent of the total traffic); and Traffic Condition C, more and larger trucks. 

In Exhibit 3-55, smaller vehicles in combination are assumed for deriving Case II widths 
than for deriving Case III widths, because passing of stalled vehicles in the former is apt to be 
very infrequent. Moreover, full offtracking need not be assumed for both the stalled and the 
passing vehicles. Often the stalled vehicles can be drifted adjacent to the inner edge of roadway, 
thereby providing additional clearance for the passing vehicle. 

The design vehicles or combinations of different design vehicles used in determination of 
values given in Exhibit 3-55 for the three traffic conditions, assuming full clearance for the design 
vehicles indicated, are: 



Metric | 


US Customary 


Case 


Design Traffic Condition 


Design Traffic Condition 


ABC 


ABC 


1 

II 
III 


P SU WB-12 
P-P P-SU SU-SU 
P-SU SU-SU WB-12-WB-12 


P SU WB-40 
P-P P-SU SU-SU 
P-SU SU-SU WB-40-WB-40 



The combination of letters, such as P-SU for Case II, means that the design width in this 
example allows a P design vehicle to pass a stalled SU design truck or vice versa. In assuming 
full clearance, allowance was made for the values of C as discussed. 



In negotiating roadways designed for smaller vehicles, larger vehicles will have less 
clearance and will need to use lower speeds and will demand more caution and skill by drivers, 
but there is a limit to the size of vehicles that can be operated on these narrower roadways. The 
larger vehicles that can be operated on turning roadways of the widths shown in Exhibit 3-55, but 
with partial clearance varying from about one-half the total values of C, as discussed for the 
sharper curves, to nearly full values for the flatter curves, are: 

225 



AASHTO — Geometric Design of Highways and Streets 



Metric 


US Customary 


Case Design Traffic Condition 


Design Traffic Condition 


ABC 


A B C 


I WB-12 WB-12 WB-15 

II P-SU P-WB-12 SU-WB-12 

III SU-WB-12 WB-12-WB-12 WB-15-WB-15 


WB-40 WB-40 WB-50 
P-SU P-WB-40 SU-WB-40 
SU-WB-40 WB-40-WB-40 WB-50-WB-50 



The widths in Exhibit 3-55 are subject to some modification with respect to the treatment at 
the edge, as shown at the bottom of the table. An occasional large vehicle can pass another on a 
roadway designed for small vehicles if there is space and stability outside the roadway and there 
is no barrier to prevent its occasional use. In such cases the width can be a little narrower than the 
tabulated dimension. Vertical curbs along the edge of a lane give drivers a sense of restriction, 
and occasional large vehicles have no additional space in which to maneuver; for this reason, 
such roadways should be a little wider than the values shown in Exhibit 3-55. 

When there is an adjacent stabilized shoulder, the widths for Cases II and III and under 
certain conditions for Case I on roadways on tangent may be reduced. Case II values may be 
reduced by the additional width of stabilized shoulder but not below the widths for Case I. 
Similarly, Case III values may be reduced by 0.6 m [2 ft]. Case I values for the individual design 
vehicles are recommended minimums and further reduction is not in order, even with a usable 
shoulder, except on tangents. When vertical curbs are used on both sides, the tabulated widths 
should be increased by 0.6 m [2 ft] for Cases I and III, or by 0.3 m [1 ft] for Case II, because 
stalled vehicles are passed at low speed. Where such a curb is on only one side of the roadway, 
the added width may be only 0.3 m [1 ft] for Cases I and III, and no added width is needed for 
Case 11. 

The use of Exhibit 3-55 in design is illustrated by the following example. Assume that the 
geometric layout and traffic volume for a specific turning movement are such that one-lane, one- 
way operation with provision for passing a stalled vehicle is called for (Case II), and that the 
traffic volume includes 10 to 12 percent trucks with an occasional large semitrailer combination 
for which traffic condition C is deemed applicable. Then, with a radius of 50 m [165 ft] for the 
inner edge of the traveled way, the width tabulated in Exhibit 3-55 is 7.0 m [23 ft]. With a 1.2-m 
[4-ft] stabilized shoulder, the turning roadway width may be reduced to 5.8 [19 ft] (see lower part 
of Exhibit 3-55). With a vertical curb on each side, the tuming roadway width should be not less 
than 7.3 m [24 ft]. 



Widths Outside Traveled Way 

The roadway width for a tuming roadway includes the shoulders or equivalent lateral 
clearance outside the traveled way. Over the whole range of intersections, the appropriate 
shoulder width varies from none, or minimal, on curbed urban streets to the width of an open- 
highway cross section. The more general cases are discussed in the following paragraphs. 



226 



Elements of Design 



Within a ciiannelized intersection, shoulders for turning roadways are usually unnecessary. 
The lanes may be defined by curbs, pavement markings, or islands. The islands may be curbed 
and the general dimensional controls for islands provide the appropriate lateral clearances outside 
the edges of the turning roadv^ay. In most instances, the turning roadways are relatively short, and 
shoulder sections are not needed for the temporary storage of vehicles. A discussion of island 
dimensions can be found in Chapter 9. 

Where there is a separate roadway for right turns, its left edge defines one side of the 
triangular island. If the island is small or especially important in directing movements, it may be 
defined by both curbs or pavement markings. On the other hand, where the turning radius is large, 
the side of the island may be defined by guideposts, by delineators, or simply by pavement 
markings and the edge of the pavement of the turning roadway. In any case, a developed left 
shoulder is normally unnecessary. However, there should be either an offset, if curbs are used, or 
a fairly level section of sufficient width on the left to avoid affecting the lateral placement of 
vehicles. 

A shoulder usually is provided on the right side of a right-turning roadway in rural areas. In 
cross section and general treatment, the right shoulder should be essentially the same as the 
shoulder of the adjacent open-highway section, possibly somewhat reduced in width because of 
conditions at the intersections. Because turning vehicles have a tendency to encroach on the 
shoulder, consideration should be given to providing heavy-duty right shoulders to accommodate 
the associated wheel loads. Although a curb on the right side might be advantageous in reducing 
maintenance operations that result from vehicles hugging the inside of the curve and causing edge 
depressions or raveling, the introduction of curbing adjacent to high-speed highways should be 
discouraged. For low-speed urban conditions, curbing of the right edge of a turning roadway is 
normal practice. Curbs are discussed in greater detail in Chapter 4. 

On large-scale channelized layouts and at interchanges, there may be turning roadways of 
sufficient curvature and length to be well removed from other roadways. Such turning roadways 
should have a shoulder on both sides. Curbs, when used, should be located at the outside edge of 
the shoulder and should be sloping. 

Some turning roadways, particularly ramps, pass over drainage structures, pass over or under 
other roadways, or pass adjacent to walls or rock cuts on one or both sides. For such locations, the 
minimum clearances for structures, as established in later chapters and in the current edition of 
the AASHTO bridge specifications (31), apply directly. In addition, the design should be 
evaluated for adequate sight distance, as the sharp curve may need above-minimum lateral 
clearance. 

Exhibit 3-56 is a summary of the range of design values for the general turning roadway 
conditions described above. On roadways without curbs or with sloping curbs, the adjacent 
shoulder should be of the same type and cross section as that on the approach highway. The 
widths shown are for usable shoulders. Where roadside barriers are provided, the width indicated 
should be measured to the face of the barrier, and the graded width should be about 0.6 m [2.0 ft] 
greater. For other than low-volume conditions, it is desirable that right shoulders be surfaced, 
surface treated, or otherwise stabilized for a width of 1.2 m [4.0 ft] or more. 

227 



AASHTO — Geometric Design of Highways and Streets 



Metric 



US Customary 



Turning roadway 
condition 



Shoulder width or lateral 

clearance outside of traveled 

way edge (m) 



Shoulder width or lateral 
clearance outside of traveled 
way edge (ft) 



Left 



Right 



Left 



Right 



Short length, usually 
within channelized 
intersection 

Intermediate to long 
length or in cut or on fill 



0.6 to 1.2 



1.2 to 3.0 



0.6 to 1 .2 



1 .8 to 3.6 



2 to 4 



4 to 10 



2 to 4 



6 to 12 



Note: All dimensions should be increased, where necessary, for sight distance. 



Exhibit 3-56<. Range of Usable Shoulder Widths or Equivalent Lateral Clearances 
Outside of Turning Roadways^ Not on Structure 



Sight Distance on Horizontal Curves 

Another element of horizontal alignment is the sight distance across the inside of curves. 
Where there are sight obstructions (such as walls, cut slopes, buildings, and longitudinal barriers) 
on the inside of curves or the inside of the median lane on divided highways, a design may need 
adjustment in the normal highway cross section or change in the alignment if removal of the 
obstruction is impractical to provide adequate sight distance. Because of the many variables in 
alignment, in cross section, and in the number, type, and location of potential obstructions, 
specific study is usually needed for each individual curve. With sight distance for the design 
speed as a control, the designer should check the actual conditions on each curve and make the 
appropriate adjustments to provide adequate sight distance. 



Stopping Sight Distance 

For general use in design of a horizontal curve, the sight line is a chord of the curve, and the 
stopping sight distance is measured along the centerline of the inside lane around the curve. 
Exhibit 3-57 is a design chart showing the middle ordinates needed for clear sight areas that 
satisfy stopping sight distance criteria presented in Exhibit 3-1 for horizontal curves of various 
radii. Exhibit 3-57 includes radii for all superelevation rates to a maximum of 12 percent. 

The middle-ordinate values in Exhibit 3-57 are derived from geometry for the several 
dimensions, as indicated in the diagrammatic sketch in Exhibit 3-58 and in Equation (3-40). The 
equation applies only to circular curves longer than the sight distance for the pertinent design 
speed. The relationships between R, M, and V in this chart can be quickly checked. For example, 
with an 80 km/h [50 mph] design speed and a curve with a 350 m [1,150 ft] radius, a clear sight 
area with a middle ordinate of approximately 6.0 m [20 ft] is needed for stopping sight distance. 
As another example, for a sight obstruction at a distance M equal to 6.0 m [20 ft] from the 
centerline of theinside lane on a curve with a 175-m [575-ft] radius, the sight distance needed is 
approximately at the upper end of the range for a speed of approximately 60 km/h [40 mph]. 



228 



Elements of Design 



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Exhibit 3-57. Design Controls for Stopping Siglit Distance on Horizontal Curves 



229 



AASHTO — Geometric Design of Highways and Streets 



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Exhibit 3-57. Design Controls for Stopping Sight Distance on Horizontal Curves 

(Continued) 



230 



Elements of Design 



Sight DiStonce (S)^ 




Exhibit 3-58o Diagram Illiistratieg CompoBents for DetermiEing Horizontal Sight Distance 



Metric 



US CystQmary 



M=R 



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where: 

S = 
R = 

M = 



where: 



Stopping sight distance, m; 
Radius of curve, m; 
Middle ordinate, m 



S = Stopping sight distance, ft; 
R = Radius of curve, ft; 
M = Middle ordinate, ft 



Horizontal sight restrictions may occur where there is a cut slope on the inside of the curve. 
For the 1,080-mm [3.5-ft] eye height and the 600-nim [2.0-ft] object height used for stopping 
sight distance, a height of 840 mm [2.75 ft] may be used as the midpoint of the sight line where 
the cut slope usually obstructs sight. This assumes that there is little or no vertical curvature. For 
a highway with a 6.6"m [22-ft] traveled way, 1.2-m [4-ft] shoulders, an allowance of 1.2 m [4 ft] 
for a ditch section, and 1V:2H (1 m or 1 ft vertically for each, 2 m or 2 ft horizontally) cut slopes, 
the sight obstruction is about 5.75 m [19 ft] outside the centerline of the inside lane. This is 
sufficient for adequate sight distance at 50 km/h [30 mph] when curves have a radius of about 90 
m [275 ft] or more and at 80 km/h [50 mph] when curves have a radius of about 375 m [1,230 ft] 
or more. Curves sharper than these would need flatter slopes, benching, or other adjustments. At 
the other extreme, highways with normal lateral dimensions of more than 16 m [52 ft] provide 



231 



AASHTO — Geometric Design of Highways and Streets 



adequate stopping sight distances for horizontal curves over the entire range of design speeds and 
curves. 

In some instances, retaining walls, concrete median safety barriers, and other similar features 
constructed on the inside of curves may be sight obstructions and should be checked for stopping 
sight distance. As an example, an obstruction of this type, located L2 m [4 ft] from the inside 
edge of a 7.2-m [24-ft] traveled way, has a middle ordinate of about 3.0 m [10 ft]. At 80 km/h 
[50 mph], this provides sufficient sight distance when a curve has a radius of about 700 m 
[2,300 ft] or more. If the obstruction is moved an additional 0.3 m [1 ft] away from the roadway 
creating a middle ordinate of 3.3 m [11 ft], a curve with a radius of 625 m [2,000 ft] or more 
provides sufficient sight distance at the same 80 km/h [50 mph] speed. The same finding would 
be applicable to existing buildings or similar sight obstructions on the inside of curves. 

Where sufficient stopping sight distance is not available because a railing or a longitudinal 
barrier constitutes a sight obstruction, alternative designs should be considered for both safety and 
economic reasons. The alternatives are: (1) increase the offset to the obstruction, (2) increase the 
radius, or (3) reduce the design speed. However, the alternative selected should not incorporate 
shoulder widths on the inside of the curve in excess of 3.6 m [12 ft] because of the concem that 
drivers will use wider shoulders as a passing or travel lane. 

As can be seen from Exhibit 3-58, the method presented is only exact when both the vehicle 
and the sight obstruction are located within the limits of the simple horizontal curve. When either 
the vehicle or the sight obstruction is situated beyond the limits of the simple curve, the values 
obtained are only approximate. The same is true if either the vehicle, the sight obstruction, or 
both is situated within the limits of a spiral or a compound curve. In these instances, the value 
obtained would result in middle ordinate values slightly larger than those needed to satisfy the 
desired stopping sight distance. In many instances, the resulting additional clearance will not be 
significant. Whenever Exhibit 3-57 is not applicable, the design should be checked either by 
utilizing graphical procedures or by utilizing a computational method. Reference (32) provides a 
computational method for making such checks. 



Passing Sight Distance 

The minimum passing sight distance for a two-lane road or street is about four times as great 
as the minimum stopping sight distance at the same design speed. To conform to those greater 
sight distances, clear sight areas on the inside of curves should have widths greatly in excess of 
those discussed. Equation (3-40) is directly applicable to passing sight distance but are of limited 
practical value except on long curves. A chart demonstration using this equation would be of 
value primarily in reaching negative conclusions — that it would be difficult to maintain passing 
sight distance on other than very flat curves. 

Passing sight distance is measured between an eye height of 1,080 mm [3.5 ft] and an object 
height of 1,080 mm [3.5 ft]. The sight line near the center of the area inside a curve is about 
240 mm [0.75 ft] higher than for stopping sight distance. The resultant lateral dimension for 
normal highway cross sections (1V:2H to 1V:6H backslopes) in cut between the centerline of the 

232 



Elements of Design 



inside lane and the midpoint of the sight line is from 0.5 to 1.5 m [1.5 to 4.5 ft] greater than that 
for stopping sight distance. It is obvious that for many cut sections, design for passing sight 
distance should, for practical reasons, be limited to tangents and very flat curves. Even in level 
terrain, provision of passing sight distance would need a clear area inside each curve that v^ould, 
in some instances, extend beyond the normal right-of-way line. 

][n general, the designer should use graphical methods to check sight distance on horizontal 
curves. This method is presented in Exhibit 3-8 and described in the accompanying discussion. 



General Controls for Horizontal Alignment 

In addition to the specific design elements for horizontal alignment discussed under previous 
headings, a number of general controls are recognized in practice. These controls are not subject 
to theoretical derivation, but they are important for efficient and smooth-flowing highways. 
Excessive curvature or poor combinations of curvature limit capacity, cause economic losses 
because of increased travel time and operating costs, and detract from a pleasing appearance. To 
avoid such poor design practices, the general controls that follow should be used where practical: 

® Alignment should be as directional as practical, but should be consistent with the 
topography and with preserving developed properties and community values. A flowing 
line that conforms generally to the natural contours is preferable to one with long 
tangents that slashes through the terrain. With curvilinear alignment, construction scars 
can be kept to a minimum and natural slopes and growth can be preserved. Such design 
is desirable from a construction and maintenance standpoint. In general, the number of 
short curves should be kept to a minimum. Winding alignment composed of short 
curves should be avoided because it usually leads to erratic operation. Although the 
aesthetic qualities of curving alignment are important, long tangents are needed on 
two-lane highways so that sufficient passing sight distance is available on as great a 
percentage of the highway length as practical. 

® In alignment developed for a given design speed, the minimum radius of curvature for 
that speed should be avoided wherever practical. The designer should attempt to use 
generally flat curves, saving the minimum radius for the most critical conditions. In 
general, the central angle of each curve should be as small as the physical conditions 
permit, so that the highway will be as directional as practical. This central angle should 
be absorbed in the longest practical curve, but on two-lane highways the exception 
noted in the preceding paragraph applies. 

® Consistent alignment should always be sought. Sharp curves should not be introduced 
at the ends of long tangents. Sudden changes from areas of flat curvature to areas of 
sharp curvature should be avoided. Where sharp curvature is introduced, it should be 
approached, where practical, by a series of successively sharper curves. 

® For small deflection angles, curves should be sufficiently long to avoid the appearance 
of a kink. Curves should be at least 150 m [500 ft] long for a central angle of 5 degrees, 
and the minimum length should be increased 30 m [100 ft] for each 1 -degree decrease 
in the central angle. The minimum length for horizontal curves on main highways, 
Lcmin, should be about three times the design speed expressed in km/h [15 times the 

233 



AASHTO — Geometric Design of Highways and Streets 



design speed expressed in mph], or Lc min=3V [15V]. On high speed controlled-access 
facilities that use flat curvature, for aesthetic reasons, the desirable minimum length for 
curves should be about double the minimum length described above, or Lc des= 6V 
[30V]. 

Sharp curvature should be avoided on long, high fills. In the absence of cut slopes, 
shrubs, and trees that extend above the level of the roadway, it is difficult for drivers to 
perceive the extent of curvature and adjust their operation accordingly. 
Caution should be exercised in the use of compound circular curves. While the use of 
compound curves affords flexibility in fitting the highway to the terrain and other 
ground controls, the ease with which such curves can be used may tempt the designer to 
use them without restraint. Preferably their use should be avoided where curves are 
sharp. Compound curves with large differences in radius introduce the same problems 
that arise at tangent approaches to circular curves. Where topography or right-of-way 
restrictions make their use appropriate, the radius of the flatter circular arc, Ri, should 
not be more than 50 percent greater than the radius of the sharper circular arc, R2 
(i.e., Ri should not exceed 1.5 R2). A multiple compound curve (i.e., several curves in 
sequence) may be suitable as a transition to sharp curves as discussed in the previous 
section on "Compound Circular Curves." A spiral transition between flat curves and 
sharp curves may be desirable. On one-way roads, such as ramps, the difference in radii 
of compound curves is not so important if the second curve is flatter than the first. 
However, the use of compound curves on ramps, with a flat curve between two sharper 
curves, is not good practice. 

Abrupt reversals in alignment should be avoided. Such changes in alignment make it 
difficult for drivers to keep within their own lane. It is also difficult to superelevate both 
curves adequately, and erratic operation may result. The distance between reverse 
curves should be the sum of the superelevation runoff lengths and the tangent runout 
lengths or, preferably, an equivalent length with spiral curves, as defined in the section 
on "Transition Design Controls" in this chapter. If sufficient distance (i.e., more than 
100 m [300 ft]) is not available to permit the tangent runout lengths or preferably an 
equivalent length with spiral to return to a normal crown section, there may be a long 
length where the centerline and the edges of roadway are at the same elevation and poor 
transverse drainage can be expected. In this case, the superelevation runoff lengths 
should be increased until they adjoin, thus providing one instantaneous level section. 
For traveled ways with straight cross slopes, there is less difficulty in returning the 
edges of roadway to a normal section and the 100-m [300-ft] guideline discussed above 
may be decreased. 

The "broken-back" or "flat-back" arrangement of curves (with a short tangent between 
two curves in the same direction) should be avoided except where very unusual 
topographical or right-of-way conditions make other alternatives impractical. Except on 
circumferential highways, most drivers do not expect successive curves to be in the 
same direction; the preponderance of successive curves in opposite directions may 
develop a subconscious expectation among drivers that makes successive curves in the 
same direction unexpected. Broken-back alignments are also not pleasing in 
appearance. Use of spiral transitions or compound curve alignments, in which there is 
some degree of continuous superelevation, is preferable for such situations. The term 
"broken-back" usually is not applied when the connecting tangent is of considerable 



234 



Elements of Design 



length. Even in this case, the alignment may be unpleasant in appearance when both 

curves are clearly visible for some distance ahead. 

To avoid the appearance of inconsistent distortion, the horizontal alignment should be 

coordinated carefully with the profile design. General controls for this coordination are 

discussed in the section of this chapter on "Combination of Horizontal and Vertical 

Alignment." 



VERTICAL ALIGNMENT 

Terrain 

The topography of the land traversed has an influence on the alignment of roads and streets. 
Topography affects horizontal alignment, but has an even more pronounced effect on vertical 
alignment. To characterize variations in topography, engineers generally separate it into three 
classifications according to terrain. 

In level terrain, highway sight distances, as governed by both horizontal and vertical 
restrictions, are generally long or can be made to be so without construction difficulty or major 
expense. 

In rolling terrain, natural slopes consistently rise above and fall below the road or street 
grade, and occasional steep slopes offer some restriction to normal horizontal and vertical 
roadway alignment. 

In mountainous terrain, longitudinal and transverse changes in the elevation of the ground 
with respect to the road or street are abrupt, and benching and side hill excavation are frequently 
needed to obtain acceptable horizontal and vertical alignment. 

Terrain classifications pertain to the general character of a specific route corridor. Routes in 
valleys, passes, or mountainous areas that have all the characteristics of roads or streets traversing 
level or rolling terrain should be classified as level or rolling. In general, rolling terrain generates 
steeper grades than level terrain, causing trucks to reduce speeds below those of passenger cars; 
mountainous terrain has even greater effects, causing some trucks to operate at crawl speeds. 



Grades 

Roads and streets should be designed to encourage uniform operation throughout. As 
discussed earlier in this chapter, design speeds are used as a means toward this end by correlation 
of various geometric features of the road or street. Design criteria have been determined for many 
highway features, but few conclusions have been reached on the appropriate relationship of 
roadway grades to design speed. Vehicle operating characteristics on grades are discussed and 
established relationships of grades and their lengths to design speed are developed below. 



235 



AASHTO — Geometric Design of Highways and Streets 



Vehicle Operating Characteristics on Grades 

Passenger cars* The practices of passenger car drivers on grades vary greatly, but it is 
generally accepted that nearly all passenger cars can readily negotiate grades as steep as 4 to 

5 percent without an appreciable loss in speed below that normally maintained on level roadways, 
except for cars with high weight/power ratios, including some compact and subcompact cars. 

Studies show that, under uncongested conditions, operation on a 3-percent upgrade, has only 
a slight effect on passenger car speeds compared to operations on the level. On steeper upgrades, 
speeds decrease progressively with increases in the grade. On downgrades, passenger car speeds 
generally are slightly higher than on level sections, but local conditions govern. 

Trucks. The effect of grades on truck speeds is much more pronounced than on speeds of 
passenger cars. The average speed of trucks on level sections of highway approximates the 
average speed of passenger cars. Trucks generally increase speed by up to about 5 percent on 
downgrades and decrease speed by 7 percent or more on upgrades as compared to their operation 
on the level. On upgrades, the maximum speed that can be maintained by a truck is dependent 
primarily on the length and steepness of the grade and the truck's weight/power ratio, which is 
the gross vehicle weight divided by the net engine power. Other factors that affect the average 
truck speed on a grade are the entering speed, the aerodynamic resistance, and skill of the driver. 
The last two factors cause only minor variations in the average speed. 

Extensive studies of truck performance have been conducted to determine the separate and 
combined effects of roadway grade, tractive effort, and gross vehicle weight (33, 34, 35, 36, 37, 

38,39). 

The effect of rate and length of grade on the speed of a typical heavy truck is shown in 
Exhibits 3-59 and 3-60. From Exhibit 3-59 it can be determined how far a truck, starting its climb 
from any speed up to approximately 120 km/h [70 mph], travels up various grades or 
combinations of grades before a certain or uniform speed is reached. For instance, with an 
entering speed of approximately 110 km/h [70 mph], the truck travels about 950 m [2,700 ft] up a 

6 percent grade before its speed is reduced to 60 km/h [35 mph]. If the entering speed is 60 km/h 
[35 mph], the speed at the end of a 300-m [1,000-ft] climb is about 43 km/h [26 mph]. This is 
determined by starting on the curve for a 6 percent grade corresponding to 60 km/h [35 mph] for 
which the distance is 750 m [2,500 ft], and proceeding along it to the point where the distance is 
300 m [1,000 ft] more, or 1,050 m [3,500 ft], for which the speed is about 43 km/h [26 mph]. 
Exhibit 3-60 shows the performance on grade for a truck that approaches the grade at or below 
crawl speed. The truck is able to accelerate to a speed of 40 km/h [25 mph] or more only on 
grades of less than 3.5 percent. These data serve as a valuable guide for design in appraising the 
effect of trucks on traffic operation for a given set of profile conditions. 

Travel time (and, therefore, speed) of trucks on grades is directly related to the weight/power 
ratio. Trucks of the same weight/power ratio typically have similar operating characteristics. 
Hence, this ratio is of considerable assistance in anticipating the performance of trucks. Normally, 



236 



Elements of Design 



METRIC 



120 

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Exhibit 3-59o Speed-Distance Curves for a Typical Heavy Truck of 120 kg/kW [200 Ib/hp] 

for Deceleration on Upgrades 



237 



AASHTO — Geometric Design of Highways and Streets 



METIRIC 




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Exhibit 3-60o Speed-Distance Curves for Acceleration of a Typical Heavy Truck 
of 120 kg/kW [200 Ib/hp] on Upgrades and Downgrades 



238 



Elements of Design 



the weight/power ratio is expressed in terms of gross weight and net power, in units of kg/kW 
[wt/hp]; while the metric unit kg is a unit of mass, rather than weight, it is commonly used to 
represent the weight of object. It has been found that trucks with weight/power ratios of about 
120 kg/kW [200 Ib/hp] have acceptable operating characteristics from the standpoint of the 
highway user. Such a weight/power ratio assures a minimum speed of about 60 km/h [35 mph] on 
a 3 percent upgrade. There is evidence that the automotive industry would find a weight/power 
ratio of this magnitude acceptable as a minimum goal in the design of commercial vehicles. There 
is also evidence that carrier operators are voluntarily recognizing this ratio as the minimum 
performance control in the loads placed on trucks of different power, the overall result being that 
weight/power ratio of trucks on highways has improved in recent years. Ratios developed from 
information obtained in conjunction with the nationwide brake performance studies conducted 
between 1949 and 1985 show, for example, that for a gross vehicle weight of 18,000 kg 
[40,000 lb], the average weight/power ratio decreased from about 220 kg/kW [360 Ib/hp] in 1949, 
to about 130 kg/kW [210 Ib/hp] in 1975; the weight/power ratio continued to fall to about 
80 kg/kW [130 Ib/hp] in 1985. This decreased weight/power ratio means greater power and better 
climbing ability for trucks on upgrades. 

There is a trend toward larger and heavier trucks with as many as three trailer units allowed 
on certain highways in some States. Studies indicate that as the number of axles increases, the 
weight/power ratio increases. Taking all factors into account, it appears conservative to use a 
weight/power ratio of 120 kg/kW [200 Ib/hp] in determining critical length of grade. However, 
there are locations where a weight/power ratio as high as 120 kg/kW [200 Ib/hp] is not 
appropriate. Where this occurs, designers are encouraged to utilize either a more representative 
weight/power ratio or an alternate method that more closely fits the conditions. 

Recreational vehicles. Consideration of recreational vehicles on grades is not as critical as 
consideration of trucks. However, on certain routes such as designated recreational routes, where 
a low percentage of trucks may not warrant a truck climbing lane, sufficient recreational vehicle 
traffic may indicate a need for an additional lane. This can be evaluated by using the design charts 
in Exhibit 3-61 in the same manner as for trucks described in the preceding section of this 
chapter. Recreational vehicles include self-contained motor homes, pickup campers, and towed 
trailers of numerous sizes. Because the characteristics of recreational vehicles vary so much, it is 
difficult to establish a single design vehicle. However, a recent study on the speed of vehicles on 
grades included recreational vehicles (40). The critical vehicle was considered to be a vehicle 
pulling a travel trailer, and the charts in Exhibits 3-61 for a typical recreational vehicle is based 
on that assumption. 



Control Grades for Design 

Maximum grades. On the basis of the data in Exhibits 3-59 through 3-62, and according to 
the grade controls now in use in a large number of States, reasonable guidehnes for maximum 
grades for use in design can be established. Maximum grades of about 5 percent are considered 



239 



AASHTO — Geometric Design of Highways and Streets 



METRIC 



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Exhibit 3-6 L Speed-Distance Curves for a Typical Recreational Vehicle on the 

Selected Upgrades (40) 



240 



Elements of Design 



METRIC 



4000 




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Exhibit 3-62* Crash Involvemeet Rate of Trucks for Which Running Speeds Are Reduced 
Below Average Running Speed of AH Traffic (41) 



241 



AASHTO — Geometric Design of Highways and Streets 



appropriate for a design speed of 110 km/h [70 mph]. For a design speed of 50 km/h [30 mph], 
maximum grades generally are in the range of 7 to 12 percent, depending on terrain. If only the 
more important highways are considered, it appears that maximum grades of 7 or 8 percent are 
representative of current design practice for a 50-km/h [30-mph] design speed. Control grades for 
design speeds from 60 to 100 km/h [40 to 60 mph] fall between the above extremes. Maximum 
grade controls for each functional class of highway and street are presented in Chapters 5 
through 8. 

The maximum design grade should be used only infrequently; in most cases, grades should 
be less than the maximum design grade. At the other extreme, for short grades less than 150 m 
[500 ft] in length and for one-way downgrades, the maximum grade may be about 1 percent 
steeper than other locations; for low-volume rural highways, the maximum grade may be 
2 percent steeper. 

Mioimiim grades. Flat grades can typically be used without problem on uncurbed highways 
where the cross slope is adequate to drain the pavement surface laterally. With curbed highways 
or streets, longitudinal grades should be provided to facilitate surface drainage. An appropriate 
minimum grade is typically 0.5 percent, but grades of 0.30 percent may be used where there is a 
high-type pavement accurately sloped and supported on firm subgrade. Use of even flatter grades 
may be justified in special cases as discussed in subsequent chapters. Particular attention should 
be given to the design of storm water inlets and their spacing to keep the spread of water on the 
traveled way within tolerable limits. Roadside channels and median swales frequently need 
grades steeper than the roadway profile for adequate drainage. Drainage channels are discussed in 
Chapter 4. 



Critical Lengths of Grade for Design 

Maximum grade in itself is not a complete design control. It is also appropriate to consider 
the length of a particular grade in relation to desirable vehicle operation. The term "critical length 
of grade" is used to indicate the maximum length of a designated upgrade on which a loaded 
truck can operate without an unreasonable reduction in speed. For a given grade, lengths less than 
critical result in acceptable operation in the desired range of speeds. If the desired freedom of 
operation is to be maintained on grades longer than critical, design adjustments such as changes 
in location to reduce grades or addition of extra lanes should be considered. The data for critical 
lengths of grade should be used with other pertinent factors (such as traffic volume in relation to 
capacity) to determine where added lanes are warranted. 

To establish design values for critical lengths of grade for which gradeability of trucks is the 
determining factor, data or assumptions are needed for the following: 

1 . Size and power of a representative truck or truck combination to be used as a design 
vehicle along with the gradeability data for this vehicle: 

A loaded truck, powered so that the weight/power ratio is about 120 kg/kW 
[200 Ib/hp], is representative of the size and type of vehicle normally used for 



242 



Elements of Design 



design control for main highways. Data in Exhibits 3-59 and 3-60 apply to such a 
vehicle. 

2. Speed at entrance to critical length of grade: 

The average running speed as related to design speed can be used to approximate the 
speed of vehicles beginning an uphill climb. This estimate is, of course, subject to 
adjustment as approach conditions may determine. Where vehicles approach on 
nearly level grades, the running speed can be used directly. For a downhill approach 
it should be increased somewhat, and for an uphill approach it should be decreased. 

3. Minimum speed on the grade below in which interference to following vehicles is 
considered unreasonable: 

No specific data are available on which to base minimum tolerable speeds of trucks 
on upgrades. It is logical to assume that such minimum speeds should be in direct 
relation to the design speed. Minimum truck speeds of about 40 to 60 km/h 
[25 to 40 mph] for the majority of highways (on which design speeds are about 60 to 
100 km/h [40 to 60 mph]) probably are not unreasonably annoying to following 
drivers unable to pass on two-lane roads, if the time interval during which they are 
unable to pass is not too long. The time interval is not likely to be annoying on 
two-lane roads with volumes well below their capacities, whereas it is likely to be 
annoying on two-lane roads with volumes near capacity. Lower minimum truck 
speeds can probably be tolerated on multilane highways rather than on two-lane 
roads because there is more opportunity for and less difficulty in passing. Highways 
should be designed so that the speeds of trucks will not be reduced enough to cause 
intolerable conditions for following drivers. 

Studies show that, regardless of the average speed on the highway, the more a vehicle 
deviates from the average speed, the greater its chances of becoming involved in a crash. One 
such study (41) used the speed distribution of vehicles traveling on highways in one state, and 
related it to the crash involvement rate to obtain the rate for trucks of four or more axles operating 
on level grades. The crash involvement rates for truck speed reductions of 10, 15, 25, and 
30 km/h [5, 10, 15, and 20 mph] were developed assuming the reduction in the average speed for 
all vehicles on a grade was 30 percent of the truck speed reduction on the same grade. The results 
of this analysis are shown in Exhibit 3-62. 

A common basis for determining critical length of grade is based on a reduction in speed of 
trucks below the average running speed of traffic. The ideal would be for all traffic to operate at 
the average speed. This, however, is not practical. In the past, the general practice has been to use 
a reduction in truck speed of 25 km/h [15 mph] below the average running speed of all traffic to 
identify the critical length of grade. As shown in Exhibit 3-62, the crash involvement rate 
increases significantly when the truck speed reduction exceeds 15 km/h [10 mph] with the 
involvement rate being 2.4 times greater for a 25-km/h [15-mph] reduction than for a 15-km/h 
[10-mph] reduction. On the basis of these relationships, it is recommended that a 15-km/h 
[10-mph] reduction criterion be used as the general guide for determining critical lengths of 
grade. 

The length of any given grade that will cause the speed of a representative truck (120 kg/kW 
[200 Ib/hp]) entering the grade at 1 10 km/h [70 mph] to be reduced by various amounts below the 

243 



AASHTO — Geometric Design of Highways and Streets 



average running speed of all traffic is shown graphically in Exhibit 3-63, which is based on the 
truck performance data presented in Exhibit 3-59. The curve showing a 15-km/h [10-mph] speed 
reduction is used as the general design guide for determining the critical lengths of grade. Similar 
information on the critical length of grade for recreational vehicles may be found in Exhibit 3-64, 
which is based on the recreational vehicle performance data presented in Exhibit 3-61. 

Where the entering speed is less than 110 km/h [70 mph], as may be the case where the 
approach is on an upgrade, the speed reductions shown in Exhibits 3-63 and 3-64 will occur over 
shorter lengths of grade. Conversely, where the approach is on a downgrade, the probable 
approach speed is greater than 110 km/h [70 mph] and the truck or recreational vehicle will 
ascend a greater length of grade than shown in the exhibits before the speed is reduced to the 
values shown. 

The method of using Exhibit 3-63 to determine critical lengths of grade is demonstrated in 
the following examples. 

Assume that a highway is being designed for 100 km/h [60 mph] and has a fairly level 
approach to a 4 percent upgrade. The 15-km/h [10-mph] speed reduction curve in Exhibit 3-63 
shows the critical length of grade to be 350 m [1,200 ft]. If, instead, the design speed was 60 
km/h [40 mph], the initial and minimum tolerable speeds on the grade would be different, but for 
the same permissible speed reduction the critical length would still be 350 m [1,200 ft]. 

In another instance, the critical length of a 5 percent upgrade approached by a 500-m [1,650- 
ft] length of 2 percent upgrade is unknown. Exhibit 3-63 shows that a 2 percent upgrade of 500 m 
[1,650 ft] in length would result in a speed reduction of about 9 km/h [6 mph]. The chart further 
shows that the remaining tolerable speed reduction of 6 km/h [4 mph] would occur on 100 m 
[325 ft] of the 5 percent upgrade. 

Where an upgrade is approached on a momentum grade, heavy trucks often increase speed, 
sometimes to a considerable degree in order to make the climb in the upgrade at as high a speed 
as practical. This factor can be recognized in design by increasing the tolerable speed reduction. It 
remains for the designer to judge to what extent the speed of trucks would increase at the bottom 
of the momentum grade above that generally found on level approaches. It appears that a speed 
increase of about 10 km/h [5 mph] can be considered for moderate downgrades and a speed 
increase of 15 km/h [10 mph] for steeper grades of moderate length or longer. On this basis, the 
tolerable speed reduction with momentum grades would be 25 or 30 km/h [15 or 20 mph]. For 
example, where there is a moderate length of 4 percent downgrade in advance of a 6 percent 
upgrade, a tolerable speed reduction of 25 km/h [15 mph] can be assumed. For this case, the 
critical length of the 6 percent upgrade is about 370 m [1,250 ft]. 

The critical length of grade in Exhibit 3-63 is derived as the length of tangent grade. Where a 
vertical curve is part of a critical length of grade, an approximate equivalent tangent grade length 
should be used. Where the condition involves vertical curves of Types II and IV shown later in 
this chapter in Exhibit 3-73 and the algebraic difference in grades is not too great, the 
measurement of critical length of grade may be made between the vertical points of intersection 



244 



Elements of Design 



METRIC 




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Exhibit 3-63. Critical Lengths of Grade for Design, Assumed Typical Heavy Truck 
of 120 kg/kW [200 Ib/hp], Entering Speed = 110 km/h [70 mph] 



245 



AASHTO — Geometric Design of Highways and Streets 



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Exhibit 3-64. Critical Lengths of Grade Using an Approach Speed of 90 km/h [55 mph] for 

Typical Recreational Vehicle (40) 



246 



Elements of Design 



(VPI). Where vertical curves of Types I and III in Exhibit 3-73 are involved, about one-quarter of 
the vertical curve length should be considered as part of the grade under consideration. 

Steep downhill grades can also have a detrimental effect on the capacity and safety of 
facilities with high traffic volumes and numerous heavy trucks. Some downgrades are long and 
steep enough that some heavy vehicles travel at crawl speeds to avoid loss of control on the 
grade. Slow-moving vehicles of this type may impede other vehicles. Therefore, there are 
instances where consideration should be given to providing a truck lane for downhill traffic. 
Procedures have been developed in the HCM (14) to analyze this situation. 

The suggested design criterion for determining the critical length of grade is not intended as 
a strict control but as a guideline. In some instances, the terrain or other physical controls may 
preclude shortening or flattening grades to meet these controls. Where a speed reduction greater 
than the suggested design guide cannot be avoided, undesirable type of operation may result on 
roads with numerous trucks, particularly on two-lane roads with volumes approaching capacity 
and in some instances on multilane highways. Where the length of critical grade is exceeded, 
consideration should be given to providing an added uphill lane for slow-moving vehicles, 
particularly where volume is at or near capacity and the truck volume is high. Data in 
Exhibit 3-63 can be used along with other pertinent considerations, particularly volume data in 
relation to capacity and volume data for trucks, to determine where such added lanes are 
warranted. 

Climbing Lanes 

Climbing Lanes for Two-Lane Highways 

General. Freedom and safety of operation on two-lane highways, besides being influenced 
by the extent and frequency of passing sections, are adversely affected by heavily loaded vehicle 
traffic operating on grades of sufficient length to result in speeds that could impede following 
vehicles. In the past, provision of added climbing lanes to improve operations on upgrades has 
been rather limited because of the additional construction costs involved. However, because of 
the increasing amount of delay and the number of serious crashes occurring on grades, such lanes 
are now more commonly included in original construction plans and additional lanes on existing 
highways are being considered as safety improvement projects. The crash potential created by 
this condition is illustrated in Exhibit 3-62. 

A highway section with a climbing lane is not considered a three-lane highway, but a two- 
lane highway with an added lane for vehicles moving slowly uphill so that other vehicles using 
the normal lane to the right of the centerline are not delayed. These faster vehicles pass the slower 
vehicles moving upgrade, but not in the lane for opposing traffic, as on a conventional two-lane 
road. A separate climbing lane exclusively for slow-moving vehicles is preferred to the addition 
of an extra lane carrying mixed traffic. Designs of two-lane highways with climbing lanes are 
illustrated in Exhibits 3-65A and 3-65B. Climbing lanes are designed for each direction 
independently of the other. Depending on the alignment and profile conditions, they may not 



247 



AASHTO— Geometric Design of Highways and Streets 



overlap, as in Exhibit 3-65 A, or they may overlap, as in Exhibit 3-65B, where there is a crest with 
a long grade on each side. 



Edge of troveled woy PiM^ 




Climbing lanes 



Edge of traveled woy 




Climbing lanes overtopping on crest 



Exhibit 3-65, Climbing Lanes on Two-Lane Highways 



It is desirable to provide a climbing lane, as an added lane for the upgrade direction of a 
two-lane highway where the grade, traffic volume, and heavy vehicle volume combine to degrade 
traffic operations from those on the approach to the grade. Where climbing lanes are provided 
there has been a high degree of compliance in their use by truck drivers. 

On highways with low volumes, only an occasional car is delayed, and climbing lanes, 
although desirable, may not be justified economically even where the critical length of grade is 
exceeded. For such cases, slow-moving vehicle turnouts should be considered to reduce delay to 
occasional passenger cars from slow-moving vehicles. Turnouts are discussed in the section on 
'Methods for Increasing Passing Opportunities on Two-Lane Roads" in this chapter. 

The following three criteria, reflecting economic considerations, should be satisfied to justify 
a climbing lane: 

1. Upgrade traffic flow rate in excess of 200 vehicles per hour. 

2. Upgrade truck flow rate in excess of 20 vehicles per hour. 

3. One of the following conditions exists: 

® A 15 km/h [10 mph] or greater speed reduction is expected for a typical heavy 

truck. 
© Level-of-service E or F exists on the grade. 



248 



Elements of Design 



® A reduction of two or more levels of service is experienced when moving from the 
approach segment to the grade. 

In addition, safety considerations may justify the addition of a climbing lane regardless of 
grade or traffic volumes. 

The upgrade flow rate is determined by multiplying the predicted or existing design hour 
volume by the directional distribution factor for the upgrade direction and dividing the result by 
the peak hour factor (the peak hour and directional distribution factors are discussed in Chapter 
2). The number of upgrade trucks is obtained by multiplying the upgrade flow rate by the 
percentage of trucks in the upgrade direction. 

Trucks, As indicated in the preceding section, only one of the three conditions specified in 
criterion 3 must be met. The critical length of grade to effect a 15 km/h [10 mph] truck speed 
reduction is found using Exhibit 3-63. This critical length is compared with the length of the 
particular grade being evaluated. If the critical length of grade is less than the length of the grade 
being studied, criterion 3 is satisfied. This evaluation should be done first because, where the 
critical length of grade is exceeded, no further evaluations under criterion 3 will be needed. 

Justification for climbing lanes where the critical length of grade is not exceeded should be 
considered from the standpoint of highway capacity. The procedures used are those from the 
HCM (14) for analysis of specific grades on two-lane highways. The remaining conditions in 
criterion 3 are evaluated using these HCM procedures. The effect of trucks on capacity is 
primarily a function of the difference between the average speed of the trucks and the average 
running speed of the passenger cars on the highway. Physical dimensions of heavy trucks and 
their poorer acceleration characteristics also have a bearing on the space they need in the traffic 
stream. 

On individual grades the effect of trucks is more severe than their average effect over a 
longer section of highway. Thus, for a given volume of mixed traffic and a fixed roadway cross 
section, a higher degree of congestion is experienced on individual grades than for the average 
operation over longer sections that include downgrades as well as upgrades. Determination of the 
design service volume on individual grades should use truck factors derived from the geometries 
of the grade and the level of service selected by the highway agency for use as the basis for 
design of the highway under consideration. 

If there is no 15-km/h [10-mph] reduction in speed (i.e., if the critical length of grade is not 
exceeded), the level of service on the grade should be examined to determine if level-of-service E 
or F exists. This is done by calculating the limiting service flow rate for level-of-service D and 
comparing this rate to the actual flow rate on the grade. The actual flow rate is determined by 
dividing the hourly volume of traffic by the peak hour factor. If the actual flow rate exceeds the 
service flow rate at level-of-service D, criterion 3 is satisfied. When the actual flow rate is less 
than the limiting value, a climbing lane is not warranted by this second element of criterion 3. 

The remaining issue to examine if neither of the other elements of criterion 3 are satisfied is 
whether there is a two-level reduction in the level of service between the approach and the 

249 



AASHTO — Geometric Design of Highways and Streets 



upgrade. To evaluate this criterion, the level of service for the grade and the approach segment 
should both be determined. Since this criterion needs consideration in only a very limited number 
of cases, it is not discussed in detail here. 

The HCM (14) provides additional details and worksheets to perform the computations 
needed for analysis in the preceding criteria. This procedure is also available in computer 
software, reducing the need for manual calculations. 

Because there are so many variables involved that hardly any given set of conditions can be 
properly described as typical, a detailed analysis such as the one described is recommended 
wherever climbing lanes are being considered. 

The location where an added lane should begin depends on the speeds at which trucks 
approach the grade and on the extent of sight distance restrictions on the approach. Where there 
are no sight distance restrictions or other conditions that limit speeds on the approach, the added 
lane may be introduced on the upgrade beyond its beginning because the speed of trucks will not 
be reduced beyond the level tolerable to following drivers until they have traveled some distance 
up the grade. This optimum point for capacity would occur for a reduction in truck speed to 60 
km/h [40 mph], but a 15 km/h [10 mph] decrease in truck speed below the average running speed, 
as discussed in the preceding section on "Critical Lengths of Grade for Design," is the most 
practical reduction obtainable from the standpoint of level of service and safety. This 15-km/h 
[10-mph] reduction is the accepted basis for determining the location at which to begin climbing 
lanes. The distance from the bottom of the grade to the point where truck speeds fall to 15 km/h 
[10 mph] below the average running speed may be determined from Exhibit 3-59 or Exhibit 3-63. 
Different curves would apply for trucks with other than a weight/power ratio of 120 kg/kW [200- 
Ib/hp]. For example, assuming an approach condition on which trucks with a 120-kg/kW [200- 
Ib/hp] weight/power ratio are traveling within a flow having an average running speed of 1 10 
km/h [70 mph], the resulting 15-km/h [10-mph] speed reduction occurs at distances of 
approximately 175 to 350 m [600 to 1,200 ft] for grades varying from 7 to 4 percent. With a 
downgrade approach, these distances would be longer and, with an upgrade approach, they would 
be shorter. Distances thus determined may be used to establish the point at which a climbing lane 
should begin. Where restrictions, upgrade approaches, or other conditions indicate the likelihood 
of low speeds for approaching trucks, the added lane should be introduced near the foot of the 
grade. The beginning of the added lane should be preceded by a tapered section with a desirable 
taper ratio of 25: 1 that should be at least 90 m [300 ft] long. 

The ideal design is to extend a climbing lane to a point beyond the crest, where a typical 
truck could attain a speed that is within 15 km/h [10 mph] of the speed of the other vehicles with 
a desirable speed of at least 60 km/h [40 mph]. This may not be practical in many instances 
because of the unduly long distance needed for trucks to accelerate to the desired speed. In such 
situations, a practical point to end the added lane is where trucks can return to the normal lane 
without undue interference with other traffic — in particular, where the sight distance becomes 
sufficient to permit passing when there is no oncoming traffic or, preferably, at least 60 m [200 ft] 
beyond that point. An appropriate taper length should be provided to permit trucks to return 
smoothly to the normal lane. For example, on a highway where the passing sight distance 
becomes available 30 m [100 ft] beyond the crest of the grade, the climbing lane should extend 

250 



Elements of Design 



90 m [300 ft] beyond the crest (i.e., 30 m [100 ft] plus 60 m [200 ft]), and an additional tapered 
section with a desirable taper ratio of 50: 1 that should be at least 180 m [600 ft] long. 

A climbing lane should desirably be as wide as the through lanes. It should be so constructed 
that it can immediately be recognized as an added lane for one direction of travel. The centerline 
of the normal two-lane highway should be clearly marked, including yellow barrier lines for 
no-passing zones. Signs at the beginning of the upgrade such as "Slower Traffic Keep Right" or 
"Trucks Use Right Lane" may be used to direct slow-moving vehicles into the climbing lane. 
These and other appropriate signs and markings for climbing lanes are presented in the 
MUTCD (6). 

The cross slope of a climbing lane is usually handled in the same manner as the addition of a 
lane to a multilane highway. Depending on agency practice, this design results in either a 
continuation of the cross slope or a lane with slightly more cross slope than the adjacent through 
lane. On a superelevated section, the cross slope is generally a continuation of the slope used on 
the through lane. 

Desirably, the shoulder on the outer edge of a climbing lane should be as wide as the 
shoulder on the normal two-lane cross section, particularly where there is bicycle traffic. 
However, this may be impractical, particularly when the climbing lane is added to an existing 
highway. A usable shoulder of 1.2 m [4 ft] in width or greater is acceptable. Although not wide 
enough for a stalled vehicle to completely clear the climbing lane, a 1.2-m [4-ft] shoulder in 
combination with the climbing lane generally provides sufficient width for both the stalled 
vehicle and a slow-speed passing vehicle without need for the latter to encroach on the through 
lane. 

In summary, climbing lanes offer a comparatively inexpensive means of overcoming 
reductions in capacity and providing improved operation where congestion on grades is caused by 
slow trucks in combination with high traffic volumes. As discussed earlier in this section, 
climbing lanes also improve safety. On some existing two-lane highways, the addition of 
climbing lanes could defer reconstruction for many years or indefinitely. In a new design, 
climbing lanes could make a two-lane highway operate efficiently, whereas a much more costly 
multilane highway would be needed without them. 



Climbing Lanes on Freeways and Multilane Highways 

GeneraL Climbing lanes, although they are becoming more prevalent, have not been used as 
extensively on freeways and multilane highways as on two-lane highways, perhaps for the reason 
that multilane facilities more frequently have sufficient capacity to handle their traffic demands, 
including the typical percentage of slow-moving vehicles with high weight/power ratios, without 
being congested. Climbing lanes are generally not as easily justified on multilane facilities as on 
two-lane highways because, on two-lane facilities, vehicles following other slower moving 
vehicles on upgrades are frequently prevented by opposing traffic from using the adjacent traffic 
lane for passing, whereas there is no such impediment to passing on multilane facilities. A slow- 
moving vehicle in the normal right lane does not impede the following vehicles that can readily 

251 



AASHTO — Geometric Design of Highways and Streets 



move left to the adjacent lane and proceed without difficulty, although there is evidence that 
safety is enhanced when vehicles in the traffic stream move at the same speed. 

Because highways are normally designed for 20 years or more in the future, there is less 
likelihood that climbing lanes will be justified on multilane facilities than on two-lane roads for 
several years after construction even though they are deemed desirable for the peak hours of the 
design year. Where this is the case, there is economic advantage in designing for, but deferring 
construction of, climbing lanes on multilane facilities. In this situation, grading for the future 
climbing lane should be provided initially. The additional grading needed for a climbing lane is 
small when compared to that needed for the overall cross section. If, however, even this 
additional grading is impractical, it is acceptable, although not desirable, to use a narrower 
shoulder adjacent to the climbing lane rather than the full shoulder provided on a normal section. 

Although primarily applicable in rural areas, there are instances where climbing lanes are 
need in urban areas. Climbing lanes are particularly important for freedom of operation on urban 
freeways where traffic volumes are high in relation to capacity. On older urban freeways and 
arterial streets with appreciable grades and no climbing lanes, it is a common occurrence for 
heavy traffic, which may otherwise operate well, to platoon on grades. 

Trucks. The principal determinants of the need for climbing lanes on multilane highways 
are critical lengths of grade, effects of trucks on grades in terms of equivalent passenger-car flow 
rates, and service volumes for the desired level of service and the next poorer level of service. 

Critical length of grade has been discussed previously in this chapter. It is the length of a 
particular upgrade that reduces the speed of low-performance trucks 15 km/h [10 mph] below the 
average running speed of the remaining traffic. The critical length of grade that results in a 
15-km/h [10-mph] truck speed reduction is found using Exhibit 3-63 and is then compared to the 
length of the particular grade being examined. If the critical length of grade is less than the length 
of grade being evaluated, consideration of a climbing lane is warranted. 

In determining service volume, the passenger-car equivalent for trucks is a significant factor. 
It is generally agreed that trucks on multilane facilities have less effect in deterring following 
vehicles than on two-lane roads. Comparison of passenger-car equivalents in the HCM (14) for 
the same percent of grade, length of grade, and percent of trucks clearly illustrates the difference 
in passenger-car equivalents of trucks for two-lane and multilane facilities. 

To justify the cost of providing a climbing lane, the existence of a low level of service on the 
grade should be the criterion, as in the case of justifying climbing lanes for two-lane roads, 
because highway users will accept a higher degree of congestion (i.e., a lower level of service) on 
individual grades than over long sections of highway. As a matter of practice, the service volume 
on an individual grade should not exceed that for the next poorer level of service from that used 
for the basic design. The one exception is that the service volume for level-of-service D should 
not be exceeded. 

Generally, climbing lanes should not be considered unless the directional traffic volume for 
the upgrade is equal to or greater than the service volume for level-of-service D. In most cases 

252 



Elements of Design 



when the service volume, including trucks, is greater than 1700 vehicles per hour per lane and the 
length of the grade and the percentage of trucks are sufficient to consider climbing lanes, the 
volume in terms of equivalent passenger cars is likely to approach or even exceed the capacity. In 
this situation, an increase in the number of lanes throughout the highway section would represent 
a better investment than the provision of climbing lanes. 

Climbing lanes are also not generally warranted on four-lane highways with directional 
volumes below 1000 vehicles per hour per lane regardless of the percentage of trucks. Although a 
truck driver will occasionally pass another truck under such conditions, the inconvenience with 
this low volume is not sufficient to justify the cost of a climbing lane in the absence of 
appropriate criteria. 

The procedures in the HCM (14) should be used to consider the traffic operational 
characteristics on the grade being examined. The maximum service flow rate for the desired level 
of sei-vice, together with the flow rate for the next poorer level of service, should be determined. 
If the flow rate on the grade exceeds the service flow rate of the next poorer level of service, 
consideration of a climbing lane is warranted. In order to use the HCM procedures, the free-flow 
speed must be determined or estimated. The free-flow speed can be determined by measuring the 
mean speed of passenger cars under low to moderate flow conditions (up to 1300 passenger cars 
per hour per lane) on the facility or similar facility. 

Recent data (14, 41) indicates that the mean free-flow speed under ideal conditions for 
multilane highways ranges from 0.6 km/h [1 mph] lower than the 85th percentile speed of 
65 km/h [40 mph] to 5 km/h [3 mph] lower than the 85th percentile speed of 100 km/h [60 mph]. 
Speed limit is one factor that affects free-flow speed. Recent research (14, 41) suggests that the 
free-flow speed is approximately 11 km/h [7 mph] higher than the speed limit on facilities with 
65- and 70-km/h [40- and 45 -mph] speed limits and 8 km/h [5 mph] higher than the speed limit 
on facilities with 80- and 90-km/h [50- and 55-mph] speed limits. Analysis based on these rules 
of thumb should be used with caution. Field measurement is the recommended method of 
determining the free-flow speed, with estimation using the above procedures employed only 
when field data are not available. 

Where the grade being investigated is located on a multilane highway, other factors should 
sometimes be considered; such factors include median type, lane widths, lateral clearance, and 
access point density. These factors are accounted for in the capacity analysis procedures by 
making adjustments in the free-flow speed and are not normally a separate consideration in 
determining whether a climbing lane would be advantageous. 

For freeways, adjustments are made in traffic operational analyses using factors for restricted 
lane widths, lateral clearances, recreational vehicles, and unfamiliar driver populations. The HCM 
(14) should be used for information on considering these factors in analysis. 

Under certain circumstances there should be consideration of additional lanes to 
accommodate trucks in the downgrade direction. This is accomplished using the same procedure 
as described above and using the passenger-car equivalents for trucks on downgrades in place of 
the values for trucks and recreational vehicles on upgrades. 

253 



AASHTO — Geometric Design of Highways and Streets 



Climbing lanes on multilane roads are usually placed on the outer or right-hand side of the 
roadway as shown in Exhibit 3-66. The principles for cross slopes, for locating terminal points, 
and for designing terminal areas or tapers for climbing lanes are discussed earlier in this chapter 
in conjunction with two-lane highways; these principles are equally applicable to climbing lanes 
on multilane facilities. A primary consideration is that the location of the uphill terminus of the 
climbing lane should be at the point where a satisfactory speed is attained by trucks, preferably 
about 15 km/h [10 mph] below the average running speed of the highway. Passing sight distance 
need not be considered on multilane highways. 




Exhibit 3-66o Climbing Lane on Freeways and Multilane Highways 



Methods for Increasing Passing Opportunities on 

Two™Lane Roads 

Several highway agencies have pioneered successful methods for providing more passing 
opportunities along two-lane roads. Some of the more recognized of these methods, including 
passing lanes, turnouts, shoulder driving, and shoulder use sections are described in the FHWA 
informational guide Low Cost Methods for Improving Traffic Operations on Two-Lane Roads 
(42). A synopsis of portions of material found in this guide pertaining to these designs is 
presented in the succeeding sections. More detailed criteria for these methods are found in the 
guide. 



Passing Lanes 



An added lane can be provided in one or both directions of travel to improve traffic 
operations in sections of lower capacity to at least the same quality of service as adjacent road 

254 



Elements of Design 



sections. Passing lanes can also be provided to improve overall traffic operations on two-lane 
highways by reducing delays caused by inadequate passing opportunities over significant lengths 
of highways, typically 10 to 100 km [6 to 60 miles]. Where passing lanes are used to improve 
traffic operations over a length of road, they frequently are provided systematically at regular 
intervals. 

The location of the added lane should appear logical to the driver. The value of a passing 
lane is more obvious at locations where passing sight distance is restricted than on long tangents 
that may provide passing opportunities even without passing lanes. On the other hand, the 
location of a passing lane should recognize the need for adequate sight distance at both the lane 
addition and lane drop tapers. A minimum sight distance of 300 m 11,000 ft] on the approach to 
each taper is recommended. The selection of an appropriate location also needs to consider the 
location of intersections and high-volume driveways in order to minimize the volume of turning 
movements on a road section where passing is encouraged. Furthermore, other physical 
constraints such as bridges and culverts should be avoided if they restrict provision of a 
continuous shoulder. 

The following is a summary of the design procedure to be followed in providing passing 
sections on two-lane highways: 

1. Horizontal and vertical alignment should be designed to provide as much of the 
highway as practical with passing sight distance (see Exhibit 3-7). 

2. Where the design volume approaches capacity, the effect of lack of passing 
opportunities in reducing the level of service should be recognized. 

3. Where the critical length of grade is less than the physical length of an upgrade, 
consideration should be given to constructing added climbing lanes. The critical length 
of grade is determined as shown in Exhibits 3-63 and 3-64. 

4. Where the extent and frequency of passing opportunities made available by application 
of Criteria 1 and 3 are still too few, consideration should be given to the construction of 
passing lane sections. 

Passing-lane sections, which may be either three or four lanes in width, are constructed on 
two-lane roads to provide the desired frequency of passing zones or to eliminate interference from 
low-speed heavy vehicles, or both. Where a sufficient number and length of passing sections 
cannot be obtained in the design of horizontal and vertical alignment alone, an occasional added 
lane in one or both directions of travel may be introduced as shown in Exhibit 3-67 to provide 
more passing opportunities. Such sections are particularly advantageous in rolling terrain, 
especially where alignment is winding or the profile includes critical lengths of grade. 

In rolling terrain a highway on tangent alignment may have restricted passing conditions 
even though the grades are below critical length. Use of passing lanes over some of the crests 
provides added passing sections in both directions where they are most needed. Passing-lane 
sections should be sufficiently long to permit several vehicles in line behind a slow-moving 
vehicle to pass before returning to the normal cross section of two-lane highway 



255 



AASHTO — Geometric Design of Highways and Streets 



~ Edge of traveled way 




' Edge of traveled woy 



Tcur-lon^ Possing section on twtj-Sone highway 



-Edge of troveied woy 




Edge of troveted way - 
Three-tone Possing 3ection on two-!ane hlcjhwoy 



Exhibit 3-67. Passing Lanes Section on Two-Lane Roads 



A minimum length of 300 m [1,000 ft], excluding tapers, is needed to assure that delayed 
vehicles have an opportunity to complete at least one pass in the added lane. Where such a lane is 
provided to reduce delays at a specific bottleneck, the needed length is controlled by the extent of 
the bottleneck. A lane added to improve overall traffic operations should be long enough, over 
0.5 km [0.3 mi], to provide a substantial reduction in traffic platooning. The optimal length is 
usually 0.8 to 3.2 km [0.5 to 2.0 mi], v^ith longer lengths of added lane appropriate where traffic 
volumes are higher. The HCM (14) provides guidance in the selection of a passing lane of 
optimal length. Operational benefits typically result in reduced platooning for 5 to 15 km 
[3 to 10 miles] downstream depending on volumes and passing opportunities. After that, normal 
levels of platooning will occur until the next added lane is encountered. 

The introduction of a passing-lane section on a two-lane highway does not necessarily 
involve much additional grading. The width of an added lane should normally be the same as the 
lane widths of the two-lane highway. It is also desirable for the adjoining shoulder to be at least 
1.2 m [4 ft] wide and, whenever practical, the shoulder width in the added section should match 
that of the adjoining two-lane highway. However, a full shoulder width is not as needed on a 
passing lane section as on a conventional two-lane highway because the vehicles likely to stop are 
few and there is little difficulty in passing a vehicle with only two wheels on the shoulder. Thus, 
if the normal shoulder width on the two-lane highway is 3.0 m [10 ft], a 1.8- to 2.4-m [6- to 8-ft] 
widening of the roadbed on each side is all that may be needed. 

Four-lane sections introduced explicitly to improve passing opportunities need not be 
divided because there is no separation of opposing traffic on the two-lane portions of the 
highway. The use of a median, however, is advantageous and should be considered on highways 
carrying a total of 500 vehicles per hour or more, particularly on highways to be ultimately 
converted to a four-lane divided cross section. 

The transition tapers at each end of the added-lane section should be designed to encourage 
safe and efficient operation. The lane drop taper length should be computed from the MUTCD (6) 
formula L=0.6WS (L=Length in meters, W=Width in meters, S=Speed in km/h) or L=WS 



256 



Elements of Design 



[L=Length in ft, W=Width in ft, S=Speed in mph] while the recommended length for the lane 
addition taper is half to two-thirds of the lane drop length. 

The signing and marking of an added lane is partially addressed in the MUTCD (6), which 
indicates the appropriate centerline markings for such lanes as well as the signing and marking of 
lane drop transitions. However, the MUTCD (6) does not address signing in advance of and at the 
lane addition. A sign with the legend "Passing Lane 1 Kilometer" ["Passing Lane 1/2 Mile"] 
should be placed in advance of each added lane in order that drivers of both slow-moving 
vehicles and following vehicles can prepare to make effective use of the added lane. Additional 
signs 3 to 10 km [2 to 5 mi] in advance are also desirable because they may reduce the frustration 
and impatience of drivers following a slow-moving vehicle by assuring them that they will soon 
have an opportunity to pass. In addition, a sign should be installed at the beginning of the lane 
addition taper to encourage slower-moving vehicles to keep right. 

The transitions between the two- and three- or four-lane pavements should be located where 
the change in width is in full view of the driver. Sections of four-lane highway, particularly 
divided sections, longer than about 3 km [2 mi] may cause the driver to lose his sense of 
awareness that the highway is basically a two-lane facility. It is essential, therefore, that 
transitions from a three- or four-lane cross section back to two lanes be properly marked and 
identified with pavement markings and signs to alert the driver of the upcoming section of 
two-lane highway. An advance sign before the end of the passing lane is particularly important to 
inform drivers of the narrower roadway ahead; for more information, see the MLTTCD (6). 



Turnouts 

A turnout is a widened, unobstructed shoulder area that allows slow-moving vehicles to pull 
out of the through lane to give passing opportunities to following vehicles (42, 43). The driver of 
the slow-moving vehicle, if there are following vehicles, is expected to pull out of the through 
lane and remain in the turnout only long enough for the following vehicles to pass before 
returning to the through lane. When there are only one or two following vehicles, this maneuver 
can be accomplished without it being necessary for the driver of the vehicle in the tumout to stop. 
However, when this number is exceeded, the driver may need to stop in the tumout in order for 
all the following vehicles to pass. Turnouts are most frequently used on lower volume roads 
where long platoons are rare and in difficult terrain with steep grades where construction of an 
additional lane may not be cost effective. Such conditions are often found in mountain, coastal, 
and scenic areas where more than 10 percent of the vehicle volumes are large trucks and 
recreational vehicles. 

The recommended length of turnouts including taper is shown in Exhibit 3-68. Turnouts 
shorter than 60 m [200 ft] are not recommended even for very low approach speeds. Turnouts 
longer than 185 m [600 ft] are not recommended for high-speed roads to avoid use of the tumout 
as a passing lane. The recommended lengths are based on the assumption that slow-moving 
vehicles enter the tumout at 8 km/h [5 mph] slower than the mean speed of the through traffic. 
This length allows the entering vehicle to coast to the midpoint of the tumout without braking, 
and then, if necessary, to brake to a stop using a deceleration rate not exceeding 3 m/s^ [10 ft/s^]. 

257 



AASHTO — Geometric Design of Highways and Streets 



The recommended lengths for turnouts include entry and exit tapers. Typical entry and exit taper 
lengths range from 15 to 30 m [50 to 100 ft] (42, 43). 



Metric 


US Customary 


Approach Minimum 


Approach Minimum 


speed (km/h) length (m)* 


speed (mph) length (ftf 


30 60 


20 200 


40 60 


30 200 


50 65 


40 300 


60 85 


45 350 


70 105 


50 450 


80 135 


55 550 


90 170 


60 600 


100 185 




^ iVIaximum length should be 185 


Ti (600 ft) to avoid use of the 


turnout as a passing lane. 





Exhibit 3-68, Recommended Lengths of Tiirnouts Including Taper 



The minimum width of the turnout is 3.6 m [12 ft] with widths of 5 m [16 ft] considered 
desirable. Turnouts wider than 5 m [16 ft] are not recommended. 

A turnout should not be located on or adjacent to a horizontal or vertical curve that limits 
sight distance in either direction. The available sight distance should be at least 300 m [1,000 ft] 
on the approach to the turnout. 

Proper signing and pavement marking are also needed both to maximize turnout usage and 
assure safe operation. An edge line marking on the right side of the turnout is desirable to guide 
drivers, especially in wider turnouts. 



Shoulder Driving 

In parts of the United States, a long-standing custom has been established for slow-moving 
vehicles to move to the shoulder when another vehicle approaches from the rear and return to the 
traveled way after that following vehicle has passed. The practice generally occurs where 
adequate paved shoulders exist and, in effect, these shoulders function as continuous turnouts. 
This custom is regarded as a courtesy to other drivers requiring little or no sacrifice in speed by 
either driver. While highway agencies may want to permit such use as a means of improving 
passing opportunities without a major capital investment, they should recognize that in many 
States shoulder driving is currently prohibited by law. Thus, a highway agency considering 
shoulder driving as a passing aid may need to propose legislation to authorize such use as well as 
develop a public education campaign to familiarize drivers with the new law. 

Highway agencies should evaluate the mileage of two-lane highways with paved shoulders 
as well as their structural quality before deciding whether to allow their use as a passing aid. It 
should be recognized that, where shoulder driving becomes common, it will not be limited to 
selected sites but rather will occur anywhere on the system where paved shoulders are provided. 

258 



Elements of Design 



Another consideration is that shoulder widths of at least 3.0 m [10 ft], and preferably 3.6 m 
[12 ft], are needed. The effect that shoulder driving may have on the use of the highway by 
bicyclists should also considered. Because the practice of shoulder driving has grown up through 
local custom, no special signing to promote such use has been created. 



Shoulder Use Sections 

Another approach to providing additional passing opportunities is to permit slow-moving 
vehicles to use paved shoulders at selected sites designated by specific signing. This is a more 
limited application of shoulder use by slow-moving vehicles than shoulder driving described in 
the previous section. Typically, drivers move to the shoulder only long enough for following 
vehicles to pass and then return to the through lane. Thus, the shoulder-use section functions as an 
extended turnout. This approach enables a highway agency to promote shoulder use only where 
the shoulder is adequate to handle anticipated traffic loads and the need for more frequent passing 
opportunities has been estabhshed by the large amount of vehicle platooning. 

Shoulder use sections generally range in length from 0.3 to 5 km [0.2 to 3 mi]. Shoulder use 
should be allowed only where shoulders are at least 3.0 m [10 ft] and preferably 3.6 m [12 ft] 
wide. Adequate structural strength to support the anticipated loads along with good surface 
conditions are needed. Particular attention needs to be placed on the condition of the shoulder 
because drivers are unlikely to use a shoulder if it is rough, broken, or covered with debris. Signs 
should be erected at both the beginning and end of the section where shoulder use is allowed. 
However, since signing of shoulder-use sections is not addressed in the MUTCD (6), special 
signing should be used. 

Emergency Escape Ramps 

General 

Where long, descending grades exist or where topographic and location controls require such 
grades on new alignment, the design and construction of an emergency escape ramp at an 
appropriate location is desirable to provide a location for out-of -control vehicles, particularly 
trucks, to slow and stop away from the main traffic stream. Out-of-control vehicles are generally 
the result of a driver losing braking ability either through overheating of the brakes due to 
mechanical failure or failure to downshift at the appropriate time. Considerable experience with 
ramps constructed on existing highways has led to the design and installation of effective ramps 
that save lives and reduce property damage. Reports and evaluations of existing ramps indicate 
that they provide acceptable deceleration rates and afford good driver control of the vehicle on 
the ramp (44). 

Forces that act on every vehicle to affect the vehicle's speed include engine, braking, and 
tractive resistance forces. Engine and braking resistance forces can be ignored in the design of 
escape ramps because the ramp should be designed for the worst case, in which the vehicle is out 
of gear and the brake system has failed. The tractive resistance force contains four subclasses: 
inertial, aerodynamic, rolling, and gradient. Inertial and negative gradient forces act to maintain 

259 



AASHTO — Geometric Design of Highways and Streets 



motion of the vehicle, while rolling, positive gradient, and air resistance forces act to retard its 
motion. Exhibit 3-69 illustrates the action of the various resistance forces on a vehicle. 




f^ ™ Air resistonce 
Fj = inertia) resistonce 
Fg== Grodient resistance 
F^ ~ Rolling resistonce 



W = Gross vehicle nnass [weight] 

H = Height 

L = Length 

a = Slope ongle 



Exhibit 3-69. Forces Acting on a Vehicle in Motion 



Inertial resistance can be described as a force that resists movement of a vehicle at rest or 
maintains a vehicle in motion, unless the vehicle is acted on by some external force. Inertial 
resistance must be overcome to either increase or decrease the speed of a vehicle. Rolling and 
positive gradient resistance forces are available to overcome the inertial resistance. Rolling 
resistance is a general term used to describe the resistance to motion at the area of contact 
between a vehicle's tires and the roadway surface and is only applicable when a vehicle is in 
motion. It is influenced by the type and displacement characteristics of the surfacing material of 
the roadway. Each surfacing material has a coefficient, expressed in kg/1,000 kg [lb/1,000 lb] of 
gross vehicle weight, which determines the amount of rolling resistance of a vehicle. The values 
shown in Exhibit 3-70 for rolling resistance have been obtained from various sources throughout 
the country and are a best available estimate. 





Metric 


US Customary | 


Rolling 




Rolling 






resistance 




resistance 






(kg/1000 kg 


Equivalent 


(lb/1000 lb 


Equivalent 


Surfacing material 


GVM) 


grade (%f 


GVW) 


grade (%)" 


Portland cement 


10 


1.0 


10 


1.0 


concrete 










Asphalt concrete 


12 


1.2 


12 


1.2 


Gravel, compacted 


15 


1.5 


15 


1.5 


Earth, sandy, loose 


37 


3.7 


37 


3.7 


Crushed aggregate, 
loose 


50 


5.0 


50 


5.0 


Gravel, loose 


100 


10.0 


100 


10.0 


Sand 


150 


15.0 


150 


15.0 


Pea gravel 


250 


25.0 


250 


25.0 


^ Rolling resistance expressed as equivalent 


gradient. 







Exhibit 3-70. Rolling Resistance of Roadway Surfacing Materials 



260 



Elements of Design 



Gradient resistance is due to the effect of gravity and is expressed as the force needed to 
move the vehicle through a given vertical distance. For gradient resistance to provide a beneficial 
force on an escape ramp, the vehicle must be moving upgrade, against gravity. In the case where 
the vehicle is descending a grade, gradient resistance is negative, thereby reducing the forces 
available to slow and stop the vehicle. The amount of gradient resistance is influenced by the total 
weight of the vehicle and the magnitude of the grade. For each percent of grade, the gradient 
resistance is 10 kg/1,000 kg [10 lb/1,000 Ibl whether the grade is positive or negative. 

The remaining component of tractive resistance is aerodynamic resistance, the force 
resulting from the retarding effect of air on the various surfaces of the vehicle. Air causes a 
significant resistance at speeds above 80 km/h [50 mph], but is negligible under 30 km/h 
[20 mph]. The effect of aerodynamic resistance has been neglected in determining the length of 
the arrester bed, thus introducing a small safety factor. 



Need and Location for Emergency Escape Ramps 

Each grade has its own unique characteristics. Highway alignment, gradient, length, and 
descent speed contribute to the potential for out-of-control vehicles. For existing highways, 
operational problems on a downgrade will often be reported by law enforcement officials, truck 
drivers, or the general public. A field review of a specific grade may reveal damaged guardrail, 
gouged pavement surfaces, or spilled oil indicating locations where drivers of heavy vehicles had 
difficulty negotiating a downgrade. For existing facilities an escape ramp should be provided as 
soon as a need is established. Crash experience (or, for new facilities, crash experience on similar 
facilities) and truck operations on the grade combined with engineering judgment are frequently 
used to determine the need for a truck escape ramp. Often the impact of a potential runaway truck 
on adjacent activities or population centers will provide sufficient reason to construct an escape 
ramp. 

Unnecessary escape ramps should be avoided. For example, a second escape ramp should 
not be needed just beyond the curve that created the need for the initial ramp. 

While there are no universal guidelines available for new and existing facilities, a variety of 
factors should be considered in selecting the specific site for an escape ramp. Each location 
presents a different array of design needs; factors that should be considered include topography, 
length and percent of grade, potential speed, economics, environmental impact, and crash 
experience. Ramps should be located to intercept the greatest number of runaway vehicles, such 
as at the bottom of the grade and at intermediate points along the grade where an out-of-control 
vehicle could cause a catastrophic crash. 

A technique for new and existing facilities available for use in analyzing operations on a 
grade, in addition to crash analysis, is the Grade Severity Rating System (45). The system uses a 
predetermined brake temperature limit (260°C [SOO'^F]) to establish a safe descent speed for the 
grade. It also can be used to determine expected brake temperatures at 0.8 km [0.5 mi] intervals 
along the downgrade. The location where brake temperatures exceed the limit indicates the point 
that brake failures can occur, leading to potential runaways. 

261 



AASHTO — Geometric Design of Highways and Streets 



Escape ramps generally may be built at any practical location where the main road alignment 
is tangent. They should be built in advance of horizontal curves that cannot be negotiated safely 
by an out-of-control vehicle and in advance of populated areas. Escape ramps should exit to the 
right of the roadway. On divided multilane highways, where a left exit may appear to be the only 
practical location, difficulties may be expected by the refusal of vehicles in the left lane to yield 
to out-of-control vehicles attempting to change lanes. 

Although crashes involving runaway trucks can occur at various sites along a grade, 
locations having multiple crashes should be analyzed in detail. Analysis of crash data pertinent to 
a prospective escape ramp site should include evaluation of the section of highway immediately 
uphill, including the amount of curvature traversed and distance to and radius of the adjacent 
curve. 

An integral part of the evaluation should be the determination of the maximum speed that an 
out-of-control vehicle could attain at the proposed site. This highest obtainable speed can then be 
used as the minimum design speed for the ramp. The 130- to 140-km/h [80- to 90-mph] entering 
speed, recommended for design, is intended to represent an extreme condition and therefore 
should not be used as the basis for selecting locations of escape ramps. Although the variables 
involved make it impractical to establish a maximum truck speed warrant for location of escape 
ramps, it is evident that anticipated speeds should be below the range used for design. The 
principal factor in determining the need for an emergency escape ramp should be the safety of the 
other traffic on the roadway, the driver of the out-of-control vehicle, and the residents along and 
at the bottom of the grade. An escape ramp, or ramps if the conditions indicate the need for more 
than one, should be located wherever grades are of a steepness and length that present a 
substantial risk of runaway trucks and topographic conditions will permit construction. 



Types of Emergency Escape Ramps 

Emergency escape ramps have been classified in a variety of ways. Three broad categories 
used to classify ramps are gravity, sandpile, and arrester bed. Within these broad categories, four 
basic emergency escape ramp designs predominate. These designs are the sandpile and three 
types of arrester beds, classified by grade of the arrester bed: descending grade, horizontal grade, 
and ascending grade. These four types are illustrated in Exhibit 3-71. 

The gravity ramp has a paved or densely compacted aggregate surface, relying primarily on 
gravitational forces to slow and stop the runaway. Rolling resistance forces contribute little to 
assist in stopping the vehicle. Gravity ramps are usually long, steep, and are constrained by 
topographic controls and costs. While a gravity ramp stops forward motion, the paved surface 
cannot prevent the vehicle from rolling back down the ramp grade and jackknifing without a 
positive capture mechanism. Therefore, the gravity ramp is the least desirable of the escape ramp 
types. 

Sandpiles, composed of loose, dry sand dumped at the ramp site, are usually no more than 
120 m [400 ft] in length. The influence of gravity is dependent on the slope of the surface. The 



262 



Elements of Design 




A. Ascending grade 




0.0% Ramp 



B. Horizontal grode 




^^ ^orriL 



C, Descending grade 




D. Send pile 



Note: Profile is along the baseline of the ramp. 



Exhibit 3-71. Basic Types of Emergency Escape Ramps 



263 



AASHTO — Geometric Design of Highways and Streets 



increase in rolling resistance is supplied by loose sand. Deceleration characteristics of sandpiles 
are usually severe and the sand can be affected by weather. Because of the deceleration 
characteristics, the sandpile is less desirable than the arrester bed. However, at locations where 
inadequate space exists for another type of ramp, the sandpile may be appropriate because of its 
compact dimensions. 

Descending-grade arrester-bed escape ramps are constructed parallel and adjacent to the 
through lanes of the highway. These ramps use loose aggregate in an arrester bed to increase 
rolling resistance to slow the vehicle. The gradient resistance acts in the direction of vehicle 
movement. As a result, the descending-grade ramps can be rather lengthy because the 
gravitational effect is not acting to help reduce the speed of the vehicle. The ramp should have a 
clear, obvious return path to the highway so drivers who doubt the effectiveness of the ramp will 
feel they will be able to return to the highway at a reduced speed. 

Where the topography can accommodate, a horizontal-grade arrester-bed escape ramp is 
another option. Constructed on an essentially flat gradient, the horizontal-grade ramp relies on the 
increased rolling resistance from the loose aggregate in an arrester bed to slow and stop the out- 
of-control vehicle, since the effect of gravity is minimal. This type of ramp is longer than the 
ascending-grade arrester bed. 

The most commonly used escape ramp is the ascending-grade arrester bed. Ramp 
installations of this type use gradient resistance to advantage, supplementing the effects of the 
aggregate in the arrester bed, and generally, reducing the length of ramp needed to stop the 
vehicle. The loose material in the arresting bed increases the rolling resistance, as in the other 
types of ramps, while the gradient resistance acts downgrade, opposite to the vehicle movement. 
The loose bedding material also serves to hold the vehicle in place on the ramp grade after it has 
come to a safe stop. 

Each of the ramp types is applicable to a particular situation where an emergency escape 
ramp is desirable and should be compatible with established location and topographic controls at 
possible sites. The procedures used for analysis of truck escape ramps are essentially the same for 
each of the categories or types identified. The rolling resistance factor for the surfacing material 
used in determining the length needed to slow and stop the runaway safely is the difference in the 
procedures. 



Design Considerations 

The combination of the above external resistance and numerous internal resistance forces not 
discussed acts to limit the maximum speed of an out-of -control vehicle. Speeds in excess of 130 
to 140 km/h [80 to 90 mph] will rarely, if ever, be attained. Therefore, an escape ramp should be 
designed for a minimum entering speed of 130 km/h [80 mph], with a 140-km/h [90-mph] design 
speed being preferred. Several formulas and software programs have been developed to 
determine the runaway speed at any point on the grade. These methods can be used to establish a 
design speed for specific grades and horizontal alignments (44^ 45, 46). 



264 



Elements of Design 



The design and construction of effective escape ramps involve a nunriber of considerations as 
follows: 

® To safely stop an out-of-control vehicle, the length of the ramp should be sufficient to 
dissipate the kinetic energy of the moving vehicle. 

® The alignment of the escape ramp should be tangent or on very flat curvature to 
minimize the driver's difficulty in controlling the vehicle. 

® The width of the ramp should be adequate to accommodate more than one vehicle 
because it is not uncommon for two or more vehicles to have need of the escape ramp 
within a short time. A minimum width of 8 m [26 ft] may be all that is practical in some 
areas, though greater widths are preferred. Desirably, a width of 9 to 12 m [30 to 40 ft] 
would more adequately accommodate two or more out-of-control vehicles. Ramp 
widths less than indicated above have been used successfully in some locations where it 
was determined that a wider width was unreasonably costly or not needed. Widths of 
ramps in use range from 3.6 to 12 m [12 to 40 ft]. 

® The surfacing material used in the arrester bed should be clean, not easily compacted, 
and have a high coefficient of rolling resistance. When aggregate is used, it should be 
rounded, uncrushed, predominantly a single size, and as free from fine-size material as 
practical. Such material will maximize the percentage of voids, thereby providing 
optimum drainage and minimizing interlocking and compaction. A material with a low 
shear strength is desirable to permit penetration of the tires. The durability of the 
aggregate should be evaluated using an appropriate crush test. Pea gravel is 
representative of the material used most frequently, although loose gravel and sand are 
also used. A gradation with a top size of 40 mm [1.5 in] has been used with success in 
several States. Material conforming to the AASHTO gradation No- 57 is effective if the 
fme-sized materials is removed. 

® Arrester beds should be constructed with a minimum aggregate depth of 1 m [3 ft]. 
Contamination of the bed material can reduce the effectiveness of the arrester bed by 
creating a hard surface layer up to 300 mm [12 in] thick at the bottom of the bed. 
Therefore, an aggregate depth up to 1,100 mm [42 in] is recommended. As the vehicle 
enters the arrester bed, the wheels of the vehicle displace the surface, sinking into the 
bed material, thus increasing the rolling resistance. To assist in decelerating the vehicle 
smoothly, the depth of the bed should be tapered from a minimum of 75 mm [3 in] at 
the entry point to the full depth of aggregate in the initial 30 to 60 m [100 to 200 ft] of 
the bed. 

® A positive means of draining the arrester bed should be provided to help protect the bed 
from freezing and avoid contamination of the arrester bed material. This can be 
accomplished by grading the base to drain, intercepting water prior to entering the bed, 
underdrain systems with transverse outlets or edge drains. Geotextiles or paving can be 
used between the subbase and the bed materials to prevent infiltration of fine materials 
that may trap water. Where toxic contamination from diesel fuel or other material 
spillage is a concern, the base of the arrester bed may be paved with concrete and 
holding tanks to retain the spilled contaminants may be provided. 

® The entrance to the ramp should be designed so that a vehicle traveling at a high rate of 
speed can enter safely. As much sight distance as practical should be provided 
preceding the ramp so that a driver can enter safely. The full length of the ramp should 

265 



AASHTO — Geometric Design of Highways and Streets 



be visible to the driver. The angle of departure for the ramp should be small, usually 
5 degrees or less. An auxiliary lane may be appropriate to assist the driver to prepare to 
enter the escape ramp. The main roadway surface should be extended to a point at or 
beyond the exit gore so that both front wheels of the out-of-control vehicle will enter 
the arrester bed simultaneously; this also provides preparation time for the driver before 
actual deceleration begins. The arrester bed should be offset laterally from the through 
lanes by an amount sufficient to preclude loose material being thrown onto the through 
lanes. 

® Access to the ramp should be made obvious by exit signing to allow the driver of an 
out-of-control vehicle time to react, so as to minimize the possibility of missing the 
ramp. Advance signing is needed to inform drivers of the existence of an escape ramp 
and to prepare drivers well in advance of the decision point so that they will have 
enough time to decide whether or not to use the escape ramp. Regulatory signs near the 
entrance should be used to discourage other motorists from entering, stopping, or 
parking at or on the ramp. The path of the ramp should be delineated to define ramp 
edges and provide nighttime direction; for more information, see the MUTCD (6). 
Illumination of the approach and ramp is desirable. 

® The characteristic that makes a truck escape ramp an effective safety device also makes 
it difficult to retrieve a vehicle captured by the ramp. A service road located adjacent to 
the arrester bed is needed so tow trucks and maintenance vehicles can use it without 
becoming trapped in the bedding material. The width of this service road should be at 
least 3 m [10 ft]. Preferably this service road should be paved but may be surfaced with 
gravel. The road should be designed such that the driver of an out-of-control vehicle 
will not mistake the service road for the arrester bed. 

® Anchors, usually located adjacent to the arrester bed at 50 to 100 m [150 to 300 ft] 
intervals, are needed to secure a tow truck when removing a vehicle from the arrester 
bed. One anchor should be located about 30 m [100 ft] in advance of the bed to assist 
the wrecker in returning a captured vehicle to a surfaced roadway. The local tow-truck 
operators can be very helpful in properly locating the anchors. 

As a vehicle rolls upgrade, it loses momentum and will eventually stop because of the effect 
of gravity. To determine the distance needed to bring the vehicle to a stop with consideration of 
the rolling resistance and gradient resistance, the following simplified equation may be used (33): 



Metric 


US Customary 


254(/?±G) 


L = (3-41) 

30(R±G) 


where: 

L = length of arrester bed, m; 
V = entering velocity, km/h; 
G = percent grade divided by 1 00; 
R = rolling resistance, expressed 

as equivalent percent gradient 

divided by 1 00 (see 

Exhibit 3-70) 


where: 

L = length of arrester bed, ft; 
V = entering velocity, mph; 
G =: percent grade divided by 1 00; 
R = rolling resistance, expressed 

as equivalent percent gradient 

divided by 100 (see 

Exhibit 3-70) 



266 



Elements of Design 



For example, assume that topographic conditions at a site selected for an emergency escape 
ramp limit the ramp to an upgrade of 10 percent (G =+ 0.10). The arrester bed is to be constructed 
with loose gravel for an entering speed of 140 km/h [90 mph]. Using Exhibit 3-70, R is 
determined to be 0.10. The length of the arrester bed should be determined using the 
Equation (3-41). For this example, the length of the arrester bed is about 400 m [1,350 ft]. 

When an arrester bed is constructed using more than one grade along its length, such as 
shown in Exhibit 3-72, the speed loss occurring on each of the grades as the vehicle traverses the 
bed should be determined using the following equation: 



Metric 


US Customary 


y; 


= V^' -254 L(R±G) 


V}=V.' '30 L(R±G) (3-42) 


where: 




where: 




Vf = 


speed at end of grade, km/h; 


Vf 


= speed at end of grade, mph; 


Vi = 


entering speed at beginning 


Vi 


= entering speed at beginning 




of grade, km/h; 




of grade, mph; 


L = 


length of grade, m; 


L 


= length of grade, ft; 


R = 


rolling resistance, expressed 


R 


= rolling resistance, expressed 




as equivalent percent 




as equivalent percent 




gradient divided by 100 (see 




gradient divided by 1 00 (see 




Exhibit 3-70); 




Exhibit 3-70); 


G = 


percent grade divided by 


G 


= percent grade divided by 




100 




100 



The final speed for one section of the ramp is subtracted from the entering speed to 
determine a new entering speed for the next section of the ramp and the calculation repeated at 
each change in grade on the ramp until sufficient length is provided to reduce the speed of the 
out-of-control vehicle to zero. 



Service Rood 




-Mainline grode 



Lighting 
O Wrecker anchors 



Exhibit 3-72. Typical Emergency Escape Ramp 



267 



AASHTO — Geometric Design of Highways and Streets 



Exhibit 3-72 shows a plan and profile of an emergency escape ramp with typical 
appurtenances. 

Where the only practical location for an escape ramp will not provide sufficient length and 
grade to completely stop an out-of -control vehicle, it should be supplemented with an acceptable 
positive attenuation device. 

Where a full-length ramp is to be provided with full deceleration capability for the design 
speed, a "last-chance" device should be considered when the consequences of leaving the end of 
the ramp are serious. 

Any ramp-end treatment should be designed with care to ensure that its advantages outweigh 
the disadvantages. The risk to others as the result of an out-of-control truck overrunning the end 
of an escape ramp may be more important than the harm to the driver or cargo of the truck. The 
abrupt deceleration of an out-of-control truck may cause shifting of the load, shearing of the fifth 
wheel, or jackknifing, all with potentially harmful occurrences to the driver and cargo. 

Mounds of bedding material between 0.6 and 1.5 m [2 and 5 ft] high with 1V:1.5H slopes 
(i.e., slopes that change in elevation by one unit of length for each 1 to 5 units of horizontal 
distance) have been used at the end of ramps in several instances as the "last-chance" device. At 
least one escape ramp has been constructed with an array of crash cushions installed to prevent an 
out-of-control vehicle from leaving the end of the ramp. Furthermore, at the end of a hard- 
surfaced gravity ramp, a gravel bed or attenuator array may sufficiently immobilize a brakeless 
runaway vehicle to keep it from rolling backward and jackknifing. Where barrels are used, the 
barrels should be filled with the same material as used in the arrester bed, so that any finer 
material does not result in contamination of the bed and reduction of the expected rolling 
resistance. 



Brake Check Areas 

Turnouts or puUoff areas at the sunmnit of a grade can be used for brake -check areas or 
mandatory -stop areas to provide an opportunity for a driver to inspect equipment on the vehicle 
and to ensure the brakes are not overheated at the beginning of the descent. In addition, 
information about the grade ahead and the location of escape ramps can be provided by 
diagrammatic signing or self-service pamphlets. An elaborate design is not needed for these 
areas. A brake-check area can be a paved lane behind and separated from the shoulder or a 
widened shoulder where a truck can stop. Appropriate signing should be used to discourage 
casual stopping by the public. 



Maintenance 

After each use, aggregate arrester beds should be reshaped using power equipment to the 
extent practical and the aggregate scarified as appropriate. Since aggregate tends to compact over 
time, the bedding material should be cleaned of contaminants and scarified periodically to retain 
the retarding characteristics of the bedding material and maintain free drainage. Using power 
equipment for work in the arrester bed reduces the exposure time for the maintenance workers to 

265 



Elements of Design 



the potential that a runaway truck may need to use the facility. Maintenance of the appurtenances 
should be accomplished as appropriate. 



Vertical Curves 



General Considerations 



Vertical curves to effect gradual changes between tangent grades may be any one of the crest 
or sag types depicted in Exhibit 3-73. Vertical curves should be simple in application and should 
result in a design that is safe and comfortable in operation, pleasing in appearance, and adequate 
for drainage. The major control for safe operation on crest vertical curves is the provision of 
ample sight distances for the design speed; while research (4) has shown that vertical curves with 
limited sight distance do not necessarily experience safety problems, it is recommended that all 
vertical curves should be designed to provide at least the stopping sight distances shown in 
Exhibit 3-1. Wherever practical, more liberal stopping sight distances should be used. 
Furthermore, additional sight distance should be provided at decision points. 





TYPE 



CREST VERTICAL CURVES 




TYPE m 

G^ and G^ - Tangent grades in percent 
A = Algebraic difference in grade 
L = Length of vertical curve 




SAG VERTICAL CURVES 



Exhibit 3-73» Types of Vertical Curves 



For driver comfort, the rate of change of grade should be kept within tolerable limits. This 
consideration is most important in sag vertical curves where gravitational and vertical centripetal 
forces act in opposite directions. Appearance also should be considered in designing vertical 



269 



AASHTO— Geometric Design of Highways and Streets 



curves. A long curve has a more pleasing appearance than a short one; short vertical curves may 
give the appearance of a sudden break in the profile due to the effect of foreshortening. 

Drainage of curbed roadways on sag vertical curves (Type III in Exhibit 3-73) needs careful 
profile design to retain a grade of not less than 0.5 percent or, in some cases, 0.30 percent for the 
outer edges of the roadway. Although not desirable, flatter grades may be appropriate in some 
situations. 

For simplicity, a parabolic curve with an equivalent vertical axis centered on the vertical 
point of intersection (VPI) is usually used in roadway profile design. The vertical offsets from the 
tangent vary as the square of the horizontal distance from the curve end (point of tangency). The 
vertical offset from the tangent grade at any point along the curve is calculated as a proportion of 
the vertical offset at the VPI, which is AL/800, where the symbols are as shown in Exhibit 3-73. 
The rate of change of grade at successive points on the curve is a constant amount for equal 
increments of horizontal distance, and is equal to the algebraic difference between intersecting 
tangent grades divided by the length of curve in meters [feet], or A/L in percent per meter 
[percent per foot]. The reciprocal L/A is the horizontal distance in meters [feet] needed to make a 
1 -percent change in gradient and is, therefore, a measure of curvature. The quantity L/A, termed 
"K," is useful in determining the horizontal distance from the vertical point of curvature (VPC) to 
the high point of Type I curves or to the low point of Type III curves. This point where the slope 
is zero occurs at a distance from the VPC equal to K times the approach gradient. The value of K 
is also useful in determining minimum lengths of vertical curves for various design speeds. Other 
details on parabolic vertical curves are found in textbooks on highway engineering. 

On certain occasions, because of critical clearance or other controls, the use of asymmetrical 
vertical curves may be appropriate. Because the conditions under which such curves are 
appropriate are infrequent, the derivation and use of the relevant equations have not been 
included herein. For use in such limited instances, refer to asymmetrical curve data found in a 
number of highway engineering texts. 



Crest Vertical Curves 

Minimum lengths of crest vertical curves based on sight distance criteria generally are 
satisfactory from the standpoint of safety, comfort, and appearance. An exception may be at 
decision areas, such as sight distance to ramp exit gores, where longer lengths are needed; for 
further information, refer to the section of this chapter concerning decision sight distance. 

Exhibit 3-74 illustrates the parameters used in determining the length of a parabolic crest 
vertical curve needed to provide any specified value of sight distance. The basic equations for 
length of a crest vertical curve in terms of algebraic difference in grade and sight distance follow: 



270 



Elements of Design 



Metric 


US Customary 


When S is less than L, 


When S is less than L, 

A5' (3^43) 
100(^2 /ij +ph^} 


When S is greater than L, 

^_^^ 200(>i+A)' 
A 


When S is greater than L, 

A 


where: 

L = length of vertical curve, m; 

S = sight distance, m; 

A = algebraic difference in grades, 

percent; 
hi = height of eye above roadway 

surface, m; 
h2 = height of object above 

roadway surface, m 


where: 

L = length of vertical curve, ft; 

S =: sight distance, ft; 

A = algebraic difference in grades, 

percent; 
hi = height of eye above roadway 

surface, ft; 
h2 = height of object above roadway 

surface, ft 





-L^ngih n? crest verticci curve (L) 



Exhibit 3-74» Parameters Considered in Determiiiiiig the Length of a Crest Vertical Curve 

to Provide Sight Distance 



When the height of eye and the height of object are 1,080 mm and 600 mm [3.5 ft 
and 2.0 ft], respectively, as used for stopping sight distance, the equations become: 



271 



AASHTO — Geometric Design of Highways and Streets 



Metric 


US Customary 


When S is less than L, 

L_AS' 
658 


When S is less than L, 

^_AS' (3-45) 
2158 


When S is greater than L, 

A 


When S is greater than L, 

L = 25-2'^^ (3-46, 
A 



Design controls — stopping sight distance. The minimum lengths of vertical curves for 
different values of A to provide the minimum stopping sight distances for each design speed are 
shown in Exhibit 3-75. The solid lines give the minimum vertical curve lengths, on the basis of 
rounded values of K as determined from Equations (3-45) and (3-46). 

The short dashed curve at the lower left, crossing these lines, indicates where S = L. Note 
that to the right of the S = L line, the value of K, or length of vertical curve per percent change in 
A, is a simple and convenient expression of the design control. For each design speed this single 
value is a positive whole number that is indicative of the rate of vertical curvature. The design 
control in terms of K covers all combinations of A and L for any one design speed; thus, A and L 
need not be indicated separately in a tabulation of design value. The selection of design curves is 
facilitated because the minimum length of curve in meters [feet] is equal to K times the algebraic 
difference in grades in percent, L = KA. Conversely, the checking of plans is simplified by 
comparing all curves with the design value for K. 

Exhibit 3-76 shows the computed K values for lengths of vertical curves corresponding to 
the stopping sight distances shown in Exhibit 3-1 for each design speed. For direct use in design, 
values of K are rounded as shown in the right column. The rounded values of K are plotted as the 
solid lines in Exhibit 3-75. These rounded values of K are higher than computed values, but the 
differences are not significant. 

Where S is greater than L (lower left in Exhibit 3-75), the computed values plot as a curve 
(as shown by the dashed line for 70 km/h [45 mph]) that bends to the left, and for small values of 
A the vertical curve lengths are zero because the sight line passes over the high point. This 
relationship does not represent desirable design practice. Most States use a minimum length of 
vertical curve, expressed as either a single value, a range for different design speeds, or a function 
of A. Values now in use range from about 30 to 100 m [100 to 325 ft]. To recognize the 
distinction in design speed and to approximate the range of current practice, minimum lengths of 
vertical curves are expressed as about 0.6 times the design speed in km/h, Lniin = 0.6V, where V is 
in kilometers per hour and L is in meters, or about three times the design speed in mph, [Lmin = 
3V], where V is in miles per hour and L is in feet. These terminal adjustments show as the 
vertical lines at the lower left of Exhibit 3-75. 



272 



f^EfHiC 



Elements of Design 




100 



200 300 400 

Length of crest vertical curve (m) 



500 



600 



yscysTOM^ey 




200 4<X) 600 800 10(X» 12(KI 14m 

Length oi m&&\ vertical curve. L (ft) 



16(K) 1800 aooo 



Exhibit 3-75. Design Controls for Crest Vertical Curves— Open Road Conditions 



273 



AASHTO— Geometric Design of Highways and Streets 



Metric 


US Customary | 


Slopping Rate of vertical 




Stopping Rate of vertical 


Design sight curvature, K^ 


Design 
speed 


sight curvature, K^ 


speed distance 


distance 


(i<m/li) (m) Calculated Design 


(mph) 


(ft) Calculated Design 


20 20 0.6 1 


15 


80 3.0 3 


30 35 1.9 2 


20 


115 6.1 7 


40 50 3.8 4 


25 


155 11.1 12 


50 65 6.4 7 


30 


200 18.5 19 


60 85 11.0 11 


35 


250 29.0 29 


70 105 16.8 17 


40 


305 43.1 44 


80 130 25.7 26 


45 


360 60.1 61 


90 160 38.9 39 


50 


425 83.7 84 


100 185 52.0 52 


55 


495 113.5 114 


110 220 73.6 74 


60 


570 150.6 151 


120 250 95.0 95 


65 


645 192.8 193 


130 285 123.4 124 


70 


730 246.9 247 




75 


820 311.6 312 




80 


910 383.7 384 


^ Rate of vertical curvature, K, is the length of curve per percent algebraic difference in | 


intersecting grades (A). K = UA 







Exhibit 3-76. Design Controls for Stopping Sight Distance and for Crest Vertical Curves 

The values of K derived above when S is less than L also can be used without significant 
error where S is greater than L. As shown in Exhibit 3-75, extension of the diagonal lines to meet 
the vertical lines for minimum lengths of vertical curves results in appreciable differences from 
the theoretical only where A is small and little or no additional cost is involved in obtaining 
longer vertical curves. 

For night driving on highways without lighting, the length of visible roadway is that 
roadway that is directly illuminated by the headlights of the vehicle. For certain conditions, the 
minimum stopping sight distance values used for design exceed the length of visible roadway. 
First, vehicle headlights have limitations on the distance over which they can project the light 
intensity levels that are needed for visibility. When headlights are operated on low beams, the 
reduced candlepower at the source plus the downward projection angle significantly restrict the 
length of visible roadway surface. Thus, particularly for high-speed conditions, stopping sight 
distance values exceed road-surface visibility distances afforded by the low-beam headlights 
regardless of whether the roadway profile is level or curving vertically. Second, for crest vertical 
curves, the area forward of the headlight beam's point of tangency with the roadway surface is 
shadowed and receives only indirect illumination. 

Since the headlight mounting height (typically about 600 mm [2 ft]) is lower than the driver 
eye height used for design (1,080 mm [3.5 ft]), the sight distance to an illuminated object is 
controlled by the height of the vehicle headlights rather than by the direct line of sight. Any 
object within the shadow zone must be high enough to extend into the headlight beam to be 
directly illuminated. On the basis of Equation (3-43), the bottom of the headlight beam is about 



274 



Elements of Design 



400 mm [1.3 ft] above the roadway at a distance ahead of the vehicle equal to the stopping sight 
distance. Although the vehicle headlight system does limit roadway visibility length as mentioned 
above, there is some mitigating effect in that other vehicles, whose taillight height typically varies 
from 450 to 600 mm [1.5 to 2.0 ft], and other sizable objects receive direct lighting from 
headlights at stopping sight distance values used for design. Furthermore, drivers are aware that 
visibility at night is less than during the day, regardless of road and street design features, and 
they may therefore be more attentive and alert. 

There is a level point on a crest vertical curve of Type I (see Exhibit 3-73), but no difficulty 
with drainage on highways with curbs is typically experienced if the curve is sharp enough so that 
a minimum grade of 0.30 percent is reached at a point about 15 m [50 ft] from the crest. This 
corresponds to K of 51 m [167 ft] per percent change in grade, which is plotted in Exhibit 3-75 as 
the drainage maximum. All combinations above or to the left of this line satisfy the drainage 
criterion. The combinations below and to the right of this line involve flatter vertical curves. 
Special attention is needed in these cases to ensure proper pavement drainage near the high point 
of crest vertical curves. It is not intended that K of 51 m [167 ft] per percent grade be considered 
a design maximum, but merely a value beyond which drainage should be more carefully 
designed. 

Design controls — passing sight distaeee. Design values of crest vertical curves for passing 
sight distance differ from those for stopping sight distance because of the different sight distance 
and object height criteria. The general Equations (3-43) and (3-44) apply, but the 1,080 mm 
[3.5 ft] height of object results in the following specific formulas with the same terms as shown 
above: 



Metric 


US Customary 


When S is less than L, 
864 


When S is less than L, 

^_AS^ (3-47) 
2800 


When S is greater than L, 

A 


When S is greater than L, 
A 



For the minimum passing sight distances shown in Exhibit 3-7, the minimum lengths of crest 
vertical curves are substantially longer than those for stopping sight distances. The extent of 
difference is evident by the values of K, or length of vertical curve per percent change in A, for 
passing sight distances shown in Exhibit 3-77. These lengths are 7 to 10 times the corresponding 
lengths for stopping sight distance. 



275 



AASHTO — Geometric Design of Highways and Streets 



Metric 


US Customary | 


Rate of 






Rate of 


vertical 






vertical 


Design speed Passing sight curvature, K* 


Design speed 


Passing sight 


curvature, K* 


(km/h) distance (m) design 


(mph) 


distance (ft) 


design 


30 200 46 


20 


710 


180 


40 270 84 


25 


900 


289 


50 345 138 


30 


1090 


424 


60 410 195 


35 


1280 


585 


70 485 272 


40 


1470 


772 


80 540 338 


45 


1625 


943 


90 615 438 


50 


1835 


1203 


100 670 520 


55 


1985 


1407 


110 730 617 


60 


2135 


1628 


120 775 695 


65 


2285 


1865 


130 815 769 


70 


2480 


2197 




75 


2580 


2377 




80 


2680 


2565 


Note: *Rate of vertical curvature, K, is the length of curve per percent algebraic difference in | 


intersecting grades (A). K=L/A 









Exhibit 3-77, Design Controls for Crest Vertical Curves Based on Passing Sight Distance 



Generally, it is impractical to design crest vertical curves to provide for passing sight 
distance because of high cost where crest cuts are involved and the difficulty of fitting the 
resulting long vertical curves to the terrain, particularly for high-speed roads. Passing sight 
distance on crest vertical curves may be practical on roads with unusual combinations of low 
design speeds and gentle grades or higher design speeds with very small algebraic differences in 
grades. Ordinarily, passing sight distance is provided only at locations where combinations of 
alignment and profile do not need the use of crest vertical curves. 



Sag Vertical Curves 

At least four different criteria for establishing lengths of sag vertical curves are recognized to 
some extent. These are (1) headlight sight distance, (2) passenger comfort, (3) drainage control, 
and (4) general appearance. 

Headlight sight distance has been used directly by some agencies and for the most part is the 
basis for determining the length of sag vertical curves recommended here. When a vehicle 
traverses a sag vertical curve at night, the portion of highway lighted ahead is dependent on the 
position of the headlights and the direction of the light beam. A headlight height of 600 mm [2 ft] 
and a 1 -degree upward divergence of the light beam from the longitudinal axis of the vehicle is 
commonly assumed. The upward spread of the light beam above the 1 -degree divergence angle 
provides some additional visible length of roadway, but is not generally considered in design. The 
following equations show the relationships between S, L, and A, using S as the distance between 



276 



Elements of Design 



the vehicle and point where the 1 -degree upward angle of the light beam intersects the surface of 
the roadway: 



Metric 


us Customary 


When S is less than L, 

200[0.6 + 5(tanr)] 


When S is less than L, 

A5' (3-49) 
200[2.0 + 5(13111°)] 


or, 

L- ^^' 
120+3.55 


or, 

^_ ^^ (3-501 
400+ 3.5 S 


When S is greater than L, 

r_oc 200[0.6 + 5(tanr)] 

A 


When S is greater than L, 

r _ r 200[2.0 + 5 (tan 1° )] ( 3-51 ) 
A 


or, 


^120 + 3.55^ 
A 




or, 

L=25- 


r 400 + 3.55^ 
I ^ ) 


( 3-52 ) 


where: 

L = length of sag vertical curve, 

m; 
S = light beam distance, m; 
A = algebraic difference in 

grades, percent 


where: 

L = length of sag vertical curve, 

ft; 
S = light beam distance, ft; 
A = algebraic difference in 

grades, percent 



For overall safety on highways, a sag vertical curve should be long enough that the light 
beam distance is nearly the same as the stopping sight distance. Accordingly, it is appropriate to 
use stopping sight distances for different design speeds as the value of S in the above equations. 
The resulting lengths of sag vertical curves for the recommended stopping sight distances for 
each design speed are shown in Exhibit 3-78 with solid lines using rounded values of K as was 
done for crest vertical curves. 

The effect on passenger comfort of the change in vertical direction is greater on sag than on 
crest vertical curves because gravitational and centripetal forces are in opposite directions, rather 
than in the same direction. Comfort due to change in vertical direction is not readily measured 
because it is affected appreciably by vehicle body suspension, vehicle body weight, tire 



m 



AASHTO — Geometric Design of Highways and Streets 



METRIC 




Drainage maximum K=51 
Computed values S>L 



100 



200 300 400 

Length of sag verticaf curve (m) 



500 



600 



US CUSTOIIAH Y 




600 800 1000 1200 1400 

Length of sag vertical curve (ft) 



2000 



Exhibit 3-78. Design Controls for Sag Vertical Curves — Open Road Conditions 



278 



Elements of Design 



flexibility, and other factors. Limited attempts at such measurements have led to the broad 
conclusion that riding is comfortable on sag vertical curves when the centripetal acceleration does 
not exceed 0.3m/s^ [1 ft/s^]. The general expression for such a criterion is: 



Metric 


US Customary 


395 


L=^^ (3-53) 
46,5 


where: 

L = length of sag vertical curve, m; 
A = algebraic difference in grades, 

percent; 
V = design speed, km/h 


where: 

L = length of sag vertical curve, ft; 
A = algebraic difference in grades, 

percent; 
V = design speed, mph 



The length of vertical curve needed to satisfy this comfort factor at the various design speeds 
is only about 50 percent of that needed to satisfy the headhght sight distance criterion for the 
normal range of design conditions. 

Drainage affects design of vertical curves of Type III (see Exhibit 3-73) where curbed 
sections are used. An approximate criterion for sag vertical curves is the same as that expressed 
for the crest conditions (i.e., a minimum grade of 0.30 percent should be provided within 15 m 
[50 ft] of the level point). This criterion corresponds to K of 51 m [167 ft] per percent change in 
grade, which is plotted in Exhibit 3-78 as the drainage maximum. The drainage criterion differs 
from other criteria in that the length of sag vertical curve determined for it is a maximum, 
whereas, the length for any other criterion is a minimum. The maximum length of the drainage 
criterion is greater than the minimum length for other criteria up to 100 km/h [65 mph]. 

For general appearance of sag vertical curves, some use was formerly made of a rule-of- 
thumb for minimum curve length of 30A [lOOA] or, in Exhibit 3-78, K == 30 [K = 100]. This 
approximation is a generalized control for small or intermediate values of A. Compared with 
headlight sight distance, it corresponds to a design speed of approximately 80 km/h [50 mph]. On 
high-type highways, longer curves are appropriate to improve appearance. 

From the preceding discussion, it is evident that design controls for sag vertical curves differ 
from those for crests, and separate design values are needed. The headlight sight distance appears 
to be the most logical criterion for general use, and the values determined for stopping sight 
distances are within the limits recognized in current practice. The use of this criterion to establish 
design values for a range of lengths of sag vertical curves is recommended. As in the case of crest 
vertical curves, it is convenient to express the design control in terms of the K rate for all values 
of A. This entails some deviation from the computed values of K for small values of A, but the 
differences are not significant. Exhibit 3-79 shows the range of computed values and the rounded 
values of K selected as design controls. The lengths of sag vertical curves on the basis of the 
design speed values of K are shown by the solid lines in Exhibit 3-78. It is to be emphasized that 
these lengths are minimum values based on design speed; longer curves are desired wherever 



279 



AASHTO — Geometric Design of Highways and Streets 



practical, but special attention to drainage should be exercised where values of K in excess of 51 
[167] are used. 

Minimum lengths of vertical curves for flat gradients also are recognized for sag conditions. 
The values determined for crest conditions appear to be generally suitable for sags. Lengths of 
sag vertical curves, shown as vertical lines in Exhibit 3-78, are equal to 0.6 times the design speed 
in km/h [three times the design speed in mph]. 

Sag vertical curves shorter than the lengths computed from Exhibit 3-79 may be justified for 
economic reasons in cases where an existing feature, such as a structure not ready for 
replacement, controls the vertical profile. In certain cases, ramps may also be designed with 
shorter sag vertical curves. Fixed-source lighting is desirable in such cases. For street design, 
some engineers accept design of a sag or crest where A is about 1 percent or less without a length 
of calculated vertical curve. However, field modifications during construction usually result in 
constructing the equivalent to a vertical curve, even if short. 



Metric 


US Customary | 


Stopping 




Stopping 


Design sight Rate of vertical 


Design 


sight Rate of vertical 


speed distance curvature, K^ 


speed 
(mph) 


distance curvature, K^ 


(km/in) (m) Calculated Design 


(ft) Calculated Design 


20 20 2.1 3 


15 


80 9.4 10 


30 35 5.1 6 


20 


115 16.5 17 


40 50 8.5 9 


25 


155 25.5 26 


50 65 12.2 13 


30 


200 36.4 37 


60 85 17.3 18 


35 


250 49.0 49 


70 105 22.6 23 


40 


305 63.4 64 


80 130 29.4 30 


45 


360 78.1 79 


90 160 37.6 38 


50 


425 95.7 96 


100 185 44.6 45 


55 


495 114.9 115 


110 220 54.4 55 


60 


570 135.7 136 


120 250 62.8 63 


65 


645 156.5 157 


130 285 72.7 73 


70 


730 180.3 181 




75 


820 205.6 206 




80 


910 231.0 231 


^ Rate of vertical curvature, K, is the length of c 


urve (m) per percent algebraic difference 


intersecting grades (A). K = L7A 







Exhibit 3-79, Design Controls for Sag Vertical Curves 



Sight Distance at Undercrossings 



Sight distance on the highway through a grade separation should be at least as long as the 
minimum stopping sight distance and preferably longer. Design of the vertical alignment is the 
same as at any other point on the highway except in some cases of sag vertical curves 
underpassing a structure illustrated in Exhibit 3-80. While not a frequent problem, the structure 
fascia may cut the line of sight and limit the sight distance to less that otherwise is attainable. It is 
generally practical to provide the minimum length of sag vertical curve discussed above at grade 

280 



Elements of Design 




Sight distance (S)- 
Line of sight — \ R J 



Exhibit 3-80« Sight Distance at Undercrossings 



separation structures, and even where the recommended grades are exceeded, the sight distance 
should not need to be reduced below the minimum recommended values for stopping sight 
distance. 

For some conditions, the designer may wish to check the available sight distance at an 
undercrossing, such as at a two-lane undercrossing without ramps where it would be desirable to 
provide passing sight distance. Such checks are best made graphically on the profile, but may be 
performed through computations. 

The general equations for sag vertical curve length at undercrossings are: 

Case 1 — Sight distance greater than length of vertical curve (S>L): 



Metric 



US Customary 



r 



800 



L-25- 



C- 



h^-^h^ 



\\ 



800 



L-25- 



C- 



k +h^\ 



(3»54) 



where: 

L = length of vertical curve, m; 

S = sight distance, m; 

A = algebraic difference in grades, 

percent; 

C = vertical clearance, m; 

hi = height of eye, m; 

hz = height of object, m 



where: 

L = length of vertical curve, ft; 

S = sight distance, ft; 

A = algebraic difference in grades, 

percent; 

C = vertical clearance, ft; 

hi = height of eye, ft; 

h2 = height of object, ft 



281 



AASHTO — Geometric Design of Highways and Streets 



Case 2 — Sight distance less than length of vertical curve (S<L): 



Metric 



US Customary 



L = 



AS- 



800 



(^ fk+h. 
C ■ 



L = - 



AS- 



800 



^y 



h,+hA^ 



\ V 



JJ 



( 3-55 ) 



where: 

L ■■ 

S : 

A : 

C : 

hi : 

h2 : 



where: 



length of vertical curve, m; 

sight distance, m; 

algebraic difference in grades, 

percent; 

vertical clearance, m; 

height of eye, m; 

height of object, m 



L 


= length of vertical curve, ft; 


S 


= sight distance, ft; 


A 


= algebraic difference in grades, 




percent; 


C 


= vertical clearance, ft; 


hi 


= height of eye, ft; 


h2 


= height of object, ft 



Using an eye height of 2.4 m [8.0 ft] for a truck driver and an object height of 0.6 m [2.0 ft] 
for the tailUghts of a vehicle, the following equations can be derived: 

Case 1 — Sight distance greater than length of vertical curve (S>L): 



Metric | 


US Customary | 


L-2S- 


r800(C-1.5)^ 




L = 2S- 


^800(C-5)^ 

A 


( 3-56 ) 



Case 2 — Sight distance less than length of vertical curve (S<L): 



Metric 


US Customary 


800(C-1.5) 


AS' 
800(C-5) 


(3-57) 



General Controls for Vertical Aligoinent 

In addition to the above specific controls for vertical alignment discussed above, there are 
several general controls that should be considered in design. 

® A smooth gradeline with gradual changes, as consistent with the type of highways, 
roads, or streets and the character of terrain, should be sought for in preference to a line 
with numerous breaks and short lengths of grades. Specific design criteria are the 
maximum grade and the critical length of grade, but the manner in which they are 



282 



Elements of Design 



applied and fitted to the ten'ain on a continuous line determines the suitability and 
appearance of the finished product. 

© The 'VoUer-coaster" or the "hidden-dip" type of profile should be avoided. Such 
profiles generally occur on relatively straight horizontal alignment where the roadway 
profile closely follows a rolling natural ground line. Examples of such undesirable 
profiles are evident on many older roads and streets; they are unpleasant aesthetically 
and difficult to drive. Hidden dips may create difficulties for drivers who wish to pass 
because the passing driver may be deceived if the view of the road or street beyond the 
dip is free of opposing vehicles. Even with shallow dips, this type of profile may be 
disconcerting because the driver cannot be sure whether or not there is an oncoming 
vehicle hidden beyond the rise. This type of profile is avoided by use of horizontal 
curves or by more gradual grades. 

® Undulating gradelines, involving substantial lengths of momentum grades, should be 
evaluated for their effect on traffic operation. Such profiles permit heavy trucks to 
operate at higher overall speeds than is possible when an upgrade is not preceded by a 
downgrade, but may encourage excessive speeds of trucks with attendant conflicts with 
other traffic. 

® A "broken-back" gradeline (two vertical curves in the same direction separated by a 
short section of tangent grade) generally should be avoided, particularly in sags where 
the full view of both vertical curves is not pleasing. This effect is particularly noticeable 
on divided roadways with open median sections. 

® On long grades, it may be preferable to place the steepest grades at the bottom and 
flatten the grades near the top of the ascent or to break the sustained grade by short 
intervals of flatter grade instead of providing a uniform sustained grade that is only 
slightly below the recommended maximum. This is particularly applicable to roads and 
streets with low design speeds. 

^ Where at-grade intersections occur on roadway sections with moderate to steep grades, 
it is desirable to reduce the grade through the intersection. Such profile changes are 
beneficial for vehicles making turns and serve to reduce the potential for crashes. 

® Sag vertical curves should be avoided in cuts unless adequate drainage can be provided. 

COMBINATIONS OF HORIZONTAL AND VERTICAL ALIGNMENT 

General Considerations 

Horizontal and vertical alignment are permanent design elements for which thorough study 
is warranted. It is extremely difficult and costly to correct alignment deficiencies after a highway 
is constructed. On freeways, there are numerous controls such as multilevel structures and costly 
right-of-way. On most arterial streets, heavy development takes place along the property lines, 
which makes it impractical to change the alignment in the future. Thus, compromises in the 
alignment designs should be weighed carefully, because any initial savings may be more than 
offset by the economic loss to the public in the form of crashes and delays. 

Horizontal and vertical alignment should not be designed independently. They complement 
each other, and poorly designed combinations can spoil the good points and aggravate the 

283 



AASHTO — Geometric Design of Highways and Streets 



deficiencies of each. Horizontal alignment and profile are among the more important of the 
permanent design elements of the highway. Excellence in the design of each and of their 
combination increases usefulness and safety, encourages uniform speed, and improves 
appearance, nearly always without additional cost (IS^ 47, 48, 49, 50^ 51, 52, 53). 



General Design Controls 

It is difficult to discuss combinations of horizontal alignment and profile without reference 
to the broader issue of highway location. These subjects are interrelated and what is said about 
one is generally applicable to the other. It is assumed in this discussion that the general location 
of a facility has been fixed and that the remaining task is the development of a specific design 
harmonizing of the vertical and horizontal lines, such that the finished highway, road, or street 
will be an economical, pleasant, and safe facility on which to travel. The physical constraints or 
influences that act singly or in combination to determine the ahgnment are the character of 
roadway based on the traffic, topography, and subsurface conditions, the existing cultural 
development, likely future developments, and the location of the roadway's terminals. Design 
speed is considered in determining the general roadway location, but as design proceeds to the 
development of more detailed alignment and profile it assumes greater importance. The selected 
design speed serves to keep all elements of design in balance. Design speed determines limiting 
values for many elements such as curvature and sight distance and influences many other 
elements such as width, clearance, and maximum gradient, which are all discussed in the 
preceding parts of this chapter. 

Appropriate combinations of horizontal alignment and profile are obtained through 
engineering studies and consideration of the following general guidelines: 

® Curvature and grades should be in proper balance. Tangent alignment or flat curvature 
at the expense of steep or long grades and excessive curvature with flat grades both 
represent poor design. A logical design that offers the best combination of safety, 
capacity, ease and uniformity of operation, and pleasing appearance within the practical 
limits of terrain and area traversed is a compromise between these two extremes. 

® Vertical curvature superimposed on horizontal curvature, or vice versa, generally results 
in a more pleasing facility, but such combinations should be analyzed for their effect on 
traffic. Successive changes in profile not in combination with horizontal curvature may 
result in a series of humps visible to the driver for some distance which, as previously 
discussed, represents an undesirable condition. The use of horizontal and vertical 
alignments in combination, however, may also result in certain undesirable 
arrangements, as discussed below. 

® Sharp horizontal curvature should not be introduced at or near the top of a pronounced 
crest vertical curve. This condition is undesirable because the driver may not perceive 
the horizontal change in alignment, especially at night. The disadvantages of this 
arrangement are avoided if the horizontal curvature leads the vertical curvature (i.e., the 
horizontal curve is made longer than the vertical curve). Suitable designs can also be 
developed by using design values well above the appropriate minimum values for the 
design speed. 

284 



Elements of Design 



Somewhat related to the preceding guidehne, sharp horizontal curvature should not be 

introduced near the bottom of a steep grade approaching or near the low point of a 

pronounced sag vertical curve. Because the view of the road ahead is foreshortened, any 

horizontal curvature other than a very flat curve assumes an undesirable distorted 

appearance. Further, vehicle speeds, particularly for trucks, are often high at the bottom 

of grades, and erratic operations may result, especially at night. 

On two-lane roads and streets, the need for passing sections at frequent intervals and 

including an appreciable percentage of the length of the roadway often supersedes the 

general guidelines for combinations of horizontal and vertical alignment. In such cases, 

it is appropriate to work toward long tangent sections to assure sufficient passing sight 

distance in design. 

Both horizontal curvature and profile should be made as flat as practical at intersections 

where sight distance along both roads or streets is important and vehicles may have to 

slow or stop. 

On divided highways and streets, variation in width of median and the use of 

independent profiles and horizontal alignments for the separate one-way roadways are 

sometimes desirable. Where traffic justifies provision of four lanes, a superior design 

without additional cost generally results from such practices. 

In residential areas, the alignment should be designed to minimize nuisance to the 

neighborhood. Generally, a depressed facility makes a highway less visible and less 

noisy to adjacent residents. Minor horizontal adjustments can sometimes be made to 

increase the buffer zone between the highway and clusters of homes. 

The alignment should be designed to enhance attractive scenic views of the natural and 

manmade environment, such as rivers, rock formations, parks, and outstanding 

structures. The highway should head into, rather than away from, those views that are 

outstanding; it should fall toward those features of interest at a low elevation, and it 

should rise toward diose features best seen from below or in silhouette against the sky. 



Alignment Coordination in Design 

Coordination of horizontal alignment and profile should not be left to chance but should 
begin with preliminary design, at which time adjustments can be readily made. Although a 
specific order of study cannot be stated for all highways, a general procedure applicable to most 
facihties is described below. 

The designer should use working drawings of a size, scale, and arrangement so that he or she 
can study long, continuous stretches of highway in both plan and profile and visualize the whole 
in three dimensions. Working drawings should be of a small scale, with the profile plotted joindy 
with the plan. A continuous roll of plan-profile paper usually is suitable for this purpose. To assist 
in this visualization, there also are programs available for personal computers (PCs) that allow 
designers to view proposed vertical and horizontal alignments in three dimensions. 

After study of the horizontal alignment and profile in preliminary form, adjustments in 
either, or both, can be made jointly to obtain the desired coordination. At this stage, the designer 
should not be concerned with line calculations other than known major controls. The study should 

285 



AASHTO — Geometric Design of Highways and Streets 



be made largely on the basis of a graphical or computer analysis. The criteria and elements of 
design covered in this and the preceding chapter should be kept in mind. For the selected design 
speed, the values for controlling curvature, gradient, sight distance, and superelevation runoff 
length should be obtained and checked graphically or with a PC or CADD system. Design speed 
may have to be adjusted during the process along some sections to conform to likely variations in 
speeds of operation. This need may occur v^here noticeable changes in alignment characteristics 
are needed to accommodate unusual terrain or right-of-way controls. In addition, the general 
design controls, as enumerated separately for horizontal alignment, vertical alignment, and their 
combination, should be considered. All aspects of terrain, traffic operation, and appearance 
should be considered and the horizontal and vertical lines should be adjusted and coordinated 
before the costly and time-consuming calculations and the preparation of construction plans to 
large scale are started. 

The coordination of horizontal alignment and profile from the standpoint of appearance 
usually can be accomplished visually on the preliminary working drawings or with the assistance 
of PC programs that have been developed for this purpose. Generally, such methods result in a 
satisfactory product when applied by an experienced designer. This means of analysis may be 
supplemented by models, sketches, or images projected by a PC at locations where the 
appearance of certain combinations of line and grade is unclear. For highways with gutters, the 
effects of superelevation transitions on gutter-line profiles should be examined. This can be 
particularly significant when flat grades are involved and can result in local depressions. Slight 
shifts in profile in relation to horizontal curves can sometimes eliminate the problem. 

The procedures described above should obviously be modified for the design of typical local 
roads or streets, as compared to higher type highways. The alignment of any local road or street, 
whether for a new roadway or for reconstruction of an existing roadway, is governed by the 
existing or likely future development along it. The crossroad or street intersections and the 
location of driveways are dominant controls. Although they should be fully considered, they 
should not override the broader desirable features described above. Even for street design, it is 
desirable to work out long, flowing alignment and profile sections rather than a connected series 
of block-by-block sections. Some examples of poor and good practice are illustrated in 
Exhibit 3-81. 



OTHER ELEMENTS AFFECTING GEOMETRIC DESIGN 

In addition to the design elements discussed previously, several other elements affect or are 
affected by the geometric design of a roadway. Each of these elements is discussed only to the 
extent needed to show its relation to geometric design and how it, in turn, is affected thereby. 
Detailed design of these elements is not covered here. 



Drainage 

Highway drainage facilities carry water across the right-of-way and remove storm water 
from the roadway itself. Drainage facilities include bridges, culverts, channels, curbs, gutters, and 

286 



Elements of Design 



PIAH 



Tangent Alignment 
PROFILE 




Avoid designing little local dips in on otherwise bng» unlfarm grods. 
These dips usuoliy result from o desire to bafonce cut and fill Qod 
to reduce overhaul 

Profile with tangent olignment 
-A- 



PL^N 



PROFILE 




Short humps in the grade should be ovoided. 

Profile with curve alignment 
-8- 



PLAH 



PLAN 





PROFILE 



PROFILE 



Preferred 



A distant side view of a Song grade on tangent witl reveot every 
bump on it. 

Distant view showing bumps in profife grodeline 



This combinoiion ts deficient for two reasons. The tongen! between 
the curves is too short, ond the reverse occurs on a crest. 

Short tangent on q crest between two horizontol cun/es 



Exhibit 3-81« Aligemeet and Profile Relationships in Roadway Design (48) 



287 



AASHTO — Geometric Design of Highways and Streets 



PLAN 



PROFILE 




This combinatton presents o poor oppeoronce ~ the horizontol 
curve looks like o sharp angle. 

Shorp angle appearance 



PLAN 




When horkontot on^J verticof curves coincide, o very sotSsfoctory 
oppeorance results. 

Coinddsng curves in horizontal and vertical dimension 
-F- 



pim 



PROFILE 



When horizontol ond verticat curves oppoae. o vety satisfactory 
oppeoronce results. 



Opposing curves in horizontai and vertical dinnen^ions 
-G- 



PLAN 



Mininnum curve for the 
design speed. 




Desiroble curve for appearance 



Very Jong flat curves, even where not required by a design speed 
ond regordless of profile, also hove a pleostng appeoronce when 
the centrot ongJe is very smoll. 



Horizontal alignment with small central angles. 



Exhibit 3-81, Allgiiment and Profile RelationsMps in Roadway Design (Continued) 



288 



Elements of Design 



PLAN 




The cloasfc cose of «oor<fffvciKof> bet^^een horteontol amJ vi*rtJ<;ol 
oUgnfneht 5n «*hk:h the v^ic^s of hofliootoJ or«J vwticol curve* 
cotrcstJt, cf<mUng o rich effect of t^me-dimei^atomif S-curves 
oomposetJ o-f conv^w Q«d concove h«fs*.«!s. 

Coincidng vertices 5n horliontal and vertical dimer^slons. 



PUN 




PROFILE 



A iegitimoi« ease of c<x>rtlBiat*on: one phase is «Jktp|>«d In th® 
hodzontat ptone> but vertkes stiti coinc*d«. Th€! long tonsent m 
plan Js soflene<3l by wierttc^l curwittjr«, 

Coinciding vertices with single- phase skip 



pim 



PUN 





Weok coordinotton of horlzontol and vertical alignment 



7ht upp^r tioe i& on essompJe of poor design becouse the Qiignnfieot 
consists of hr^g torsgent, wtth sfiort C!^rv«fS, whereos the botonce 

between the curves m<i tan9«r»ts lf> the lower aiigoment ts the 
pf®f«rr#d design. 



Honiontoj Qlfgnment should be bolonced 



Exhibit 3-81o Alignment ^iid Profile RelationsMps in Roadway Design (Continued) 



289 



AASHTO — Geometric Design of Highways and Streets 



PLAN 




A disjointed effect occurs when the begtrvning of o horizontal 
curve is hidden from the driver by an intervening crest wbii« 
the continuation of th« curve is vlsibie in th« distonce beyond 
the intervening crest, 

Dtsjointed effect 




Horizontal oiignment view ~ 2 breaks moximum 



Pr?OFilE 




Vertical alignment view - 3 breaks moximum 



Guideline to be us&d for coordinotion of horizontol ond 

verticof otignment. 



Good coordination of horizontoii ond vertical alignment 



Exhibit 3»8L Aligomeet and Profile Relationships ie Roadway Design (CoetiEued) 



290 



Elements of Design 



various types of drains. Hydraulic capacities and locations of such structures should be designed 
to take into consideration damage to upstream and downstream property and to secure as low a 
degree of risk of traffic interruption by flooding as is consistent with the importance of the road, 
the design traffic service needs, Federal and State regulations, and available funds. While 
drainage design considerations are an integral part of highway geometric design, specific 
drainage design criteria are not included in this policy. The AASHTO Highway Drainage 
Guidelines (54) should be referred to for a general discussion of drainage, and the AASHTO 
Model Drainage Manual (55) should be referred to for guidelines on major areas of highway 
hydraulic design. 

Many State highway agencies have excellent highway drainage manuals that may be used 
for reference for hydraulic design procedures. Alternatively, the AASHTO Model Drainage 
Manual (55) and computer software (56) may be referenced. In addition, other publications on 
drainage are widely used and are available to highway agencies from FHWA or the National 
Technical Information Service (56). 

Hydraulic requirements for stream crossings and flood plain encroachments frequently affect 
highway alignment and profile (57). The probable effects of a highway encroachment on the risk 
of flood damage to other property and the risk of flood damage to the highway should be 
evaluated when a flood plain location is under consideration. Water surface elevations for floods 
of various return periods will influence decisions regarding the highway profile where an 
encroachment on the flood plain is considered. Highway profiles at stream crossings will often be 
determined by hydrauhc considerations. To the extent practical, stream crossings and other 
highway encroachments on flood plains should be located and aligned to preserve the natural 
flood flow distribution and direction. Stream stability and the stream environment are also 
important and complex considerations in highway location and design. 

Surface channels are used to intercept and remove surface runoff from roadways, wherever 
practical. They should have adequate capacity for the design runoff and should be properly 
located and shaped. Channels are usually lined with vegetation, and rock or paved channel linings 
are used where vegetation will not control erosion. Runoff from roadway surfaces normally 
drains down grass slopes to roadside or median channels. Curbs or dikes, inlets, and chutes or 
flumes are used where runoff from the roadway would erode fill slopes. Where storm drains are 
needed, curbs are usually provided. Care should be exercised to ensure that these curbs do not 
encroach on the clear zone of the highway. For further guidance, refer to the discussion on 
horizontal clearance to obstructions in Chapter 4. 

Drainage inlets should be designed and located to limit the spread of water on the traveled 
way to tolerable widths. Because grates may become blocked by trash accumulation, curb 
openings or combination inlets with both grate and curb openings are advantageous for urban 
conditions. Grate inlets and depressions or curb-opening inlets should be located outside the 
through-traffic lanes to minimize the shifting of vehicles attempting to avoid riding over them. 
Inlet grates should also be designed to accommodate bicycle and pedestrian traffic where 
appropriate. Discontinuous sections of curbing, as at the gore of ramps, and variable curb offsets 
should not be used as expedients to handle pavement drainage where these features would detract 
from highway safety. Inlets should be designed and located to prevent silt and debris carried in 

291 



AASHTO — Geometric Design of Highways and Streets 



suspension from being deposited on the traveled way where the longitudinal gradient is 
decreased. Extra inlets should be installed near low points of sag vertical curves to take any 
overflow from blocked inlets. Inlets should be located so that concentrated flow and heavy sheet 
flow will not cross traffic lanes. Where roadway surfaces are warped, as at cross streets or ramps, 
surface water should be intercepted just before the change in cross slope. Also, inlets should be 
located just upgrade of pedestrian crossings. Storm drains should have adequate capacity to avoid 
ponding of water on the roadway and bridges, especially in sag vertical curves. The general effect 
of drainage on the geometry of roadways, shoulder ditches, or gutters and side slopes is discussed 
further in Chapter 4. 

Drainage is usually more difficult and costly for urban than for rural highways because of 
more rapid rates and larger volumes of runoff, costlier potential damage to adjacent property by 
flooding, higher overall costs because of more inlets and underground systems, greater 
restrictions because of urban development, lack of natural areas of water bodies to receive flood 
water, and higher volumes of traffic, including pedestrians. There is greater need to intercept 
concentrated storm water before it reaches the highway and to remove over-the-curb flow and 
surface water without interrupting traffic flow or causing a problem for vehicle occupants or 
pedestrians. To accommodate such runoff, underground systems and numerous inlets, curbs, and 
gutters are usually needed. Often new outfall drains of considerable length must be constructed 
because existing storm water systems often lack capacity for highway surface drainage volumes. 
A joint use storm water system, shared by the highway agency with others, can have economic 
advantages to both parties, because it is normally more economical to build a common system 
rather than two independent systems. Urban drainage design is discussed in the FHWA Urban 
Drainage Design Manual (58). 

Reduction of peak flows can be achieved by the storage of water that falls on the site in 
detention basins, storm drainage pipes, swales and channels, parking lots, and rooftops. Storm 
water is released to the downstream conveyance facility or stream at a reduced flow rate. This 
concept should be considered for use in highway drainage design where existing downstream 
conveyance facilities are inadequate to handle peak flow rates from highway storm drainage 
facilities, where the highway would contribute to increased peak flow rates and aggravate 
downstream flooding problems, and as a technique to reduce the construction costs of outfalls 
from highway storm drainage facilities. Storm water detention may also be needed in order to 
conform with Federal and State water quality regulations. Some States have environmental 
regulations that require specific pollution/erosion measures. 

The cost of drainage is neither incidental nor minor on most roads. Careful attention to needs 
for adequate drainage and protection of the highway from floods in all phases of location and 
design will prove to be effective in reducing costs in both construction and maintenance. 



Erosion Control and Landscape Development 

Erosion prevention is one of the major factors in design, construction, and maintenance of 
highways. It should be considered early in the location and design stages. Some degree of erosion 
control can be incorporated into the geometric design, particularly in the cross section elements. 

292 



Elements of Design 



Of course, the most direct application of erosion control occurs in drainage design and in the 
writing of specifications for landscaping and slope planting. 

Erosion and maintenance are minimized largely by the use of flat side slopes, rounded and 
blended with natural terrain; serrated cut slopes; drainage channels designed with due regard to 
width, depth, slopes, alignment, and protective treatment; inlets located and spaced with erosion 
control in mind; prevention of erosion at culvert outlets; proper facilities for groundwater 
interception; dikes, berms, and other protective devices to trap sediment at strategic locations; and 
protective ground covers and planting. 

Landscape development should be in keeping with the character of the highway and its 
environment. Programs include the following general areas of improvement: (1) preservation of 
existing vegetation, (2) transplanting of existing vegetation where practical, (3) planting of new 
vegetation, (4) selective clearing and thinning, and (5) regeneration of natural plant species and 
material. 

The objectives in planting or the retention and preservation of natural growth on roadsides 
are closely related. In essence, they are to provide (1) vegetation that will be an aid to aesthetics 
and safety, (2) vegetation that will aid in lowering construction and maintenance costs, and (3) 
vegetation that creates interest, usefulness, and beauty for the pleasure and satisfaction of the 
traveling public. 

Landscaping of urban highways and streets assumes additional importance in mitigating the 
many nuisances associated with urban traffic. Landscaping can reduce this contribution to urban 
blight and make the urban highways and streets better neighbors. 

Further information concerning landscape development and erosion control is presented in 
the AASHTO Guide for Transportation Landscape and Environmental Design (49). 



Rest Areas, Information Centers^ and 
Scenic Overlooks 

Rest areas, information centers, and scenic overlooks are functional and desirable elements 
of the complete highway facility and are provided for the safety and convenience of highway 
users. A safety rest area is a roadside area, with parking facilities separated from the roadway, 
provided for the travelers to stop and rest for short periods. The area may provide drinking water, 
restrooms, tables and benches, telephones, information displays, and other facilities for travelers. 
A rest area is not intended to be used for social or civic gatherings or for such active forms of 
recreation as boating, swimming, or organized games. An information center is a staffed or 
unstaffed facility at a rest area for the purpose of furnishing travel and other information or 
services to travelers. A scenic overlook is a roadside area provided for motorists to park their 
vehicles, beyond the shoulder, primarily for viewing the scenery or for taking photographs in 
safety. Scenic overlooks need not provide comfort and convenience facilities. 



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AASHTO — Geometric Design of Highways and Streets 



Site selection for rest areas, information centers, and scenic overlooks should consider the 
scenic quality of the area, accessibility, and adaptability to development. Other essential 
considerations include an adequate source of water and a means to treat and/or properly dispose 
of sewage. Site plans should be developed through the use of a comprehensive site planning 
process that should include the location of ramps, parking areas for cars and trucks, buildings, 
picnic areas, water supply, sewage treatment facilities, and maintenance areas. The objective is to 
give maximum weight to the appropriateness of the site rather than adherence to uniform distance 
or driving time between sites. 

Facilities should be designed to accommodate the needs of older persons and persons with 
disabilities. Further information concerning rest area design is presented in the AASHTO Guide 
for Development of Rest Areas on Major Arterials and Freeways (59). 



Ligiiting 

Lighting may improve the safety of a highway or street and the ease and comfort of 
operation thereon. Statistics indicate that nighttime crash rates are higher than daytime crash 
rates. To a large extent, this may be attributed to reduced visibility at night. There is evidence that 
in urban and suburban areas, where there are concentrations of pedestrians and roadside 
intersectional interferences, fixed-source lighting tends to reduce crashes. Lighting of rural 
highways may be desirable, but the need for it is much less than on streets and highways in urban 
areas. The general consensus is that lighting of rural highways is seldom justified except in 
certain critical areas, such as interchanges, intersections, railroad grade crossings, long or narrow 
bridges, tunnels, sharp curves, and areas where roadside interferences are present. Most modem 
rural highways should be designed with an open cross section and horizontal and vertical 
alignment of a fairly high type. Accordingly, they offer an opportunity for near maximum use of 
vehicle headlights, resulting in reduced justification for fixed highway lighting. 

On freeways where there are no pedestrians, roadside entrances, or other intersections at 
grade, and where rights-of-way are relatively wide, the justification for lighting differs from that 
of noncontrolled streets and highways. The AASHTO Informational Guide for Roadway Lighting 
(60) was prepared to aid in the selection of sections of freeways, highways, and streets for which 
fixed-source lighting may be warranted, and to present design guide values for their illumination. 
This guide also contains a section on the lighting of tunnels and underpasses. 

Whether or not rural at-grade intersections should be lighted depends on the layout and the 
traffic volumes involved. Intersections that do not have channelization are frequently left 
unlighted. On the other hand, intersections with substantial channelization, particularly multi-road 
layouts and those designed on a broad scale, are often lighted. It is especially desirable to 
illuminate large-scale channelized intersections. Because of the sharp curvatures, little of such 
intersections is within the lateral range of headlights, and the headlights of other vehicles are a 
hindrance rather than an aid because of the variety of directions and turning movements. There is 
need to obtain a reduction in the speed of vehicles approaching some intersections. The indication 
of this need should be definite and visible at a distance from the intersection that is beyond the 
range of headlights. Illumination of the intersection with fixed-source lighting accomplishes this. 

294 



Elements of Design 



At interchanges it also is desirable, and sometimes essential, to provide fixed-source 
lighting. Drivers should be able to see not only the road ahead, but also the entire turning 
roadv^ay area to properly discern the paths to be followed. They should also see all other vehicles 
that may influence their ov^n behavior. Without lighting, there may be a noticeable decrease in 
the usefulness of the interchange at night; there would be more cars slowing down and moving 
with uncertainty at night than during daylight hours. Consideration should be given to improving 
visibility at night by roadway lighting (or reflectorizing devices) the parts of grade separation 
structures that particularly should be avoided by motorists, such as curbs, piers, and abutments. 
The greater the volume of traffic, particularly turning traffic, the more important the fixed-source 
lighting at interchanges becomes. Illumination should also be considered on those sections of 
major highways where there are turning movements to and from roadside development. 

Floodlighting or highway lighting may be desirable at railroad-highway grade crossings 
when there are nighttime movements of trains. In some cases, such treatments may apply also to 
crossings operated with flashing signals, or gates, or both. 

Tunnels, toll plazas, and movable bridges are nearly always lighted, as are bridges of 
substantial length in urban and suburban areas. It is questionable whether the cost of lighting long 
bridges in rural areas is justified or desirable. 

To minimize the effect of glare and to provide the most economical lighting installation, 
luminaires are mounted at heights of at least 9 m [30 ft]. Lighting uniformity is improved with 
higher mounting heights, and in most cases, mounting heights of 10 to 15 m [35 to 50 ft] are 
usually preferable. High mast lighting, special luminaires on masts of 30 m [100 ft], is used to 
light large highway areas such as interchanges and rest areas. This lighting furnishes a uniform 
light distribution over the whole area and may provide alignment guidance. However, it also has a 
disadvantage in that the visual impact on the surrounding community from scattered light is 
increased. 

Luminaire supports (poles) should be placed outside the roadside clear zones whenever 
practical. The appropriate clear zone dimensions for the various functional classifications will be 
found in the discussion of horizontal clearance to obstructions in Chapter 4. Where poles are 
located within the clear zone, regardless of distances from the traveled way, they should be 
designed to have a suitable impact attenuation feature; normally, a breakaway design is used. 
Breakaway poles should not be used on streets in densely developed areas, particularly with 
sidewalks. When struck, these poles could interfere with pedestrians and cause damage to 
adjacent buildings. Because of lower speeds and parked vehicles, there is much less chance of 
injuries to vehicle occupants from striking fixed poles on a street as compared to a highway. 
Poles should not be erected along the outside of curves on ramps where they are more susceptible 
to being struck. Poles located behind longitudinal barriers (installed for other purposes) should be 
offset sufficiently to allow for deflection of the longitudinal barriers under impact. 

On a divided highway or street, luminaire supports may be located either in the median or on 
the right side of the roadway. Where luminaire supports are located on the right side of the 
roadway, the light source is usually closer to the more heavily used traffic lanes. However, with 
median installation, the cost is generally lower and illumination is greater on the high-speed 

295 



AASHTO— Geometric Design of Highways and Streets 



lanes. For median installations, dual-mast arms should be used, for which 12 to 15 m [40 to 50 ft] 
mounting heights are favored. These should be protected with a suitable longitudinal barrier. On 
narrow medians, it is usually preferable to place the luminaire supports so they are integral with 
the median barrier. 

Where highway lighting is being considered for future installation, considerable savings can 
be effected through design and installation of necessary conduits under roadways and curbs as 
part of initial construction. 

Highway lighting for freeways is intimately associated with the type and location of highway 
signs. For full effectiveness, the two should be designed jointly. 



Utilities 

Highway and street improvements, whether upgraded within the existing right-of-way or 
entirely on new right-of-way, generally entail adjustment of utility facilities. Although utilities 
generally have little effect on the geometric design of the highway or street, full consideration 
should be given to measures, reflecting sound engineering principles and economic factors, 
needed to preserve and protect the integrity and visual quality of the highway or street, its 
maintenance efficiency, and the safety of traffic. The costs of utility adjustments vary 
considerably because of the large number of companies, type and complexity of the facility, and 
the degree of involvement with the improvement. Depending on the location of a project, the 
utilities involved could include (1) sanitary sewers; (2) water supply Hnes; (3) oil, gas, and 
petroleum product pipelines; (4) overhead and underground power and communications lines 
including fiber optic cable; (5) cable television; (6) wireless communication towers; (7) drainage 
and irrigation lines; (8) heating mains; and (9) special tunnels for building connections. 



General 

Utility lines should be located to minimize need for later adjustment, to accommodate future 
highway or street improvements, and to permit servicing such lines with minimum interference to 
traffic. 

Longitudinal installation should be located on uniform alignment as near as practical to the 
right-of-way line so as to provide a safe environment for traffic operation and preserve space for 
future highway or street improvements or other utility installations. To the extent practical, 
utilities along freeways should be constructed so they can be serviced from outside the controlled 
access lines. 

To the extent practical, utility line crossings of the highway should cross on a line generally 
normal to the highway alignment. Those utility crossings that are more likely to need future 
servicing should be encased or installed in tunnels to permit servicing without disrupting the 
traffic flow. 



296 



Elements of Design 



The horizontal and vertical location of utility lines within the highway right-of-way limits 
should conform to the clear roadside policies applicable for the system, type of highway or street, 
and specific conditions for the particular section involved. Safety of the traveling public should 
be a prime consideration in the location and design of utility facilities on highway and street 
rights-of-way. The clear roadside dimension to be maintained for a specific functional 
classification is discussed in the section on "Horizontal Clearance to Obstructions" in Chapter 4. 

Sometimes attachment of utility facilities to highway structures, such as bridges, is a 
practical arrangement and may be authorized. Where it is practical to locate utility lines 
elsewhere, attachment to bridge structures should be avoided. 

On new installations or adjustments to existing utility lines, provision should be made for 
known or planned expansion of the utility facilities, particularly those located underground or 
attached to bridges. 

All utility installations on, over, or under highway or street right-of-way and attached 
structures should be of durable materials designed for long service-life expectancy, relatively free 
from routine servicing and maintenance, and meet or exceed the applicable industry codes or 
specifications. 

Utilities that are to cross or otherwise occupy the right-of-way of rural or urban freeways 
should conform to the AASHTO Policy on the Accommodation of Utilities Within Freeway 
Right-of-Way (61). Those on non-controlled access highways and streets should conform to the 
AASHTO Guide for Accommodating Utilities Within Highway Right-of-Way (62). 

Urban 

Because of lack of space in most metropolitan areas, special consideration should be given in 
the initial highway design to the potential for joint usage of the right-of-way that is consistent 
with the primary function of the highway or street. 

Appurtenances to underground installations, such as vents, drains, markers, manholes, and 
shutoffs, should be located so as not to interfere with the safety or maintenance of the highway or 
street, and so as not to be concealed by vegetation. Preferably they should be located near the 
right-of-way line. 

Where there are curbed sections, utilities should be located in the border areas between the 
curb and sidewalk, at least 0.5 m [1.5 ft] behind the face of the curb, and where practical, behind 
the sidewalk. Where shoulders are provided rather than curbs, a clear zone commensurate with 
rural conditions should be provided. 

Existing development and limited right-of-way widths may preclude location of some or all 
utility facilities outside the roadway of the street or highway. Under some conditions, it may be 
appropriate to reserve the area outside the roadway exclusively for the use of overhead Unes with 
all other utilities located under the roadway, and in some instances the location of all the facilities 



297 



AASHTO — Geometric Design of Highways and Streets 



under the roadway may be appropriate. Location under the roadway is an exception to the stated 
pohcy and as such requires special consideration and treatment. Accommodation of these 
facilities under the roadway should be accomplished in a manner that will ensure a minimum 
adverse effect on traffic as a result of future utihty service and maintenance activities. 



Rural 

On new construction no utility should be situated under any part of the roadway, except 
where it crosses the highway. 

Normally, no poles should be located in the median of divided highways. Utility poles, vent 
standpipes, and other above-ground utility appurtenances that may be struck by errant vehicles 
should not be permitted within the highway clear zone. The only exceptions permitted would be 
where the appurtenance is breakaway or could be installed behind a traffic barrier erected to 
protect errant vehicles from some other potential risk. The AASHTO Roadside Design Guide (63) 
discusses clear zone widths and may be used as a reference to determine appropriate widths for 
freeways, rural arterials, and high-speed rural collectors. For low-speed rural collectors and rural 
local roads, except for very low-volume local roads with ADTs less than or equal to 400 vehicles 
per day, a minimum clear zone of 3 m [10 ft] should be provided. 



Traffic Control Devices 

Signing and Marking 

Signing and marking are directly related to the design of the highway or street and are 
features of traffic control and operation that the designer should consider in the geometric layout 
of such a facility. The signing and marking should be designed concurrently with the geometries. 
The potential for future operational problems can be significantly reduced if signing and marking 
are treated as an integral part of design. The extent to which signs and markings are used depends 
on the traffic volume, the type of facility, and the extent of traffic control appropriate for safe and 
efficient operation. Arterial highways are usually numbered routes of fairly high type and have 
relatively high traffic volumes. On such highways, signs and markings are employed extensively. 
Local roads are low-type highways and usually have low volumes and speeds. On these facilities 
the use of complex traffic control devices is limited. 

Although safety and efficiency of operation depend to a considerable degree on the 
geometric design of the facility, the physical layout should also be supplemented by effective 
signing as a means of informing, warning, and controlling drivers. Signing plans coordinated with 
horizontal and vertical alignment, sight distance obstructions, operational speeds and maneuvers, 
and other applicable items should be worked out before completion of design. 

Highway signs are of three general types: regulatory signs, used to indicate the rules for 
traffic movement; warning signs, used to indicate conditions that may involve risk to highway 
users; and guide signs, used to direct traffic along a route or toward a destination. Uniformity in 

298 



Elements of Design 



the use of signs and other traffic control devices is the main objective of the poHcies contained in 

the MUTCD (6). 

Location, reflectorization, and lighting of signs are important considerations in signing. For 
details regarding design, location, and application of signs, reference should be made to the 

MUTCD (6). 

Because supports for highway signs have the potential of being struck by motorists, signs 
should be placed on structures, outside the clear zone, or behind traffic barriers placed for other 
reasons. If these measures are not practical, the sign supports should be breakaway or, for 
overhead sign supports, shielded by appropriate traffic barriers. The AASHTO Standard 
Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (64) 
provides the criteria for breakaway sign supports. Likewise, sign supports should not be placed to 
block sidewalks. Sign supports in sidewalks can severely impact pedestrians with vision 
impairments and are obstacles to all pedestrians. 

Markings and markers, like signs, have the function of controlling traffic to encourage safe 
and efficient operation. Markings or markers either supplement regulatory or warning signs or 
serve independently to indicate certain regulations or warn of certain conditions present on the 
highway. For highways and streets there are three general types of markings in use — pavement 
markings, object markings, and delineators. 

Pavement markings include centerline stripes, lane lines, and edge striping. These may be 
supplemented by other pavement markings, such as approach to obstructions, stop and crosswalk 
lines, and various word and symbol markings. 

Physical obstructions in or near the roadway should be removed in order to provide the 
appropriate clear zone. Where removal is impractical, such objects should be adequately marked 
by painting or by use of other high-visibility material. Where the object is in the direct line of 
traffic, the obstruction and marking thereon preferably should be illuminated at night by 
floodlighting; where this is not practical, the object markings should be effectively reflectorized. 

Post-mounted delineators are another type of marking device used to guide traffic, 
particularly at night. Reflector units are installed at certain heights and spacings to deHneate the 
roadway where alignment changes may be confusing and not clearly defined. Refer to the 
MUTCD (6) for marking criteria, methods, and policies. 



Traffic Signals 

Traffic-control signals are devices that control vehicular and pedestrian traffic by assigning 
the right-of-way to various movements for certain pre-timed or traffic -actuated intervals of time. 
They are one of the key elements in the function of many urban streets and of some rural 
intersections. For this reason the planned signal operation for each intersection of a facility should 
be integrated with the design so as to achieve optimum operational efficiency. Careful 
consideration should be given in plan development to intersection and access locations, horizontal 

299 



AASHTO — Geometric Design of Highways and Streets 



and vertical curvature with respect to signal visibility, pedestrian needs, and geometric schematics 
to ensure effective signal operation (individual signal phasing and traffic coordination between 
signals). In addition to initial installation, potential future signal needs should also be evaluated. 
The design of traffic signal devices and warrants for their use are covered in the MUTCD (6). 

Lane arrangement is the key to successful operation of signalized intersections. The crossing 
distances for both vehicles and pedestrians should be kept as short as practical to reduce exposure 
to conflicting movements. Therefore, the first step in the development of intersection geometries 
should be a complete analysis of current and future traffic demand. The need to provide right- and 
left-turn lanes to minimize the interference of turning traffic with the movement of through traffic 
should be evaluated concurrently with the potential for obtaining any additional right-of-way 
needed. Along a highway or street with a number of signalized intersections, the locations where 
turns will, or will not, be accommodated should also be examined to ensure a good fit with two- 
way signal coordination. Because of the large volume of traffic turning into and out of large 
parking areas, parking area entrances and exits should be designed in a manner that will simplify 
the operation of the affected traffic signals. 



Noise Barriers 

In recognition of the adverse effect that noise can have on people living on, working on, or 
otherwise using land adjacent to highways, noise barriers are being used to an increasing extent. 
Such noise barriers have been constructed on both new and existing highways. 

Careful consideration should be exercised to ensure that the construction of these noise 
barriers will not compromise the safety of the highway. Every effort should be made to locate 
noise barriers to allow for sign placement and to provide the horizontal clearances to obstructions 
outside the edge-of-traveled way estabUshed in Chapter 4. It is recognized, however, that such a 
setback may sometimes be impractical. In such situations, the largest practical width 
commensurate with cost-effectiveness consideration should be provided. Stopping sight distance 
is another important design consideration. Therefore, horizontal clearances should be checked for 
adequate sight distances. Construction of a noise barrier should be avoided at a given location if it 
would limit stopping sight distance below the minimum values shown in Exhibit 3-1. This 
situation could be particularly critical where the location of the noise barrier is along the inside of 
a curve. Some designs use a concrete safety shape either as an integral part of the noise barrier or 
as a separate roadside barrier between the edge of roadway and the noise barrier. On non-tangent 
alignments a separate concrete roadside barrier may obstruct sight distance even though the noise 
barrier does not. In such instances it may be appropriate to install metal rather than concrete 
roadside barriers in order to retain adequate sight distance. Care should be exercised in the 
location of noise barriers near gore areas. Barriers at these locations should begin or terminate, as 
the case may be, at least 60 m [200 ft] from the theoretical nose. 

For further discussion on noise barriers, see the section on noise control in Chapter 4. 



300 



Elements of Design 



Fencing 

Highway agencies use fencing extensively to delineate the acquired control of access for a 
highway. While provision of fencing is not a duty, fencing may also serve to reduce the likelihood 
of encroachment onto the highway right-of-way. 

Any portion of a highway with full control of access may be fenced except in areas of 
precipitous slopes, natural barriers, or where it can be established that fencing is not needed to 
preserve access control. Fencing is usually located at or near the right-of-way line or, where 
frontage roads are used, in the area between the through highway and the frontage road (outer 
separation). 

Fencing for access control is usually owned by the highway agency so that the agency has 
control of the type and location of fence. The lowest cost type of fence best suited to the specific 
adjacent land use is generally provided. If fencing is not needed for access control, the fence 
should be the property of the adjacent landowner. 



Maintenance of Traffic Through Construction Areas 

Maintenance of a safe flow of traffic during construction should be carefully planned and 
executed. Although it is often better to provide detours, this is frequently impractical and flow of 
traffic is maintained through the construction area. Sometimes traffic lanes are closed, shifted, or 
encroached upon in order that the construction can be undertaken. When this occurs, designs for 
traffic control should minimize the effect on traffic operations by minimizing the frequency or 
duration of interference with normal traffic flow. The development of traffic control plans is an 
essential part of the overall project design and may affect the design of the facility itself The 
traffic control plan depends on the nature and scope of the improvement, volumes of traffic, 
highway or street pattern, and capacities of available highways or streets. A well-thought-out and 
carefully developed plan for the movement of traffic through a work zone will contribute 
significantly to the safe and efficient flow of traffic as well as the safety of the construction 
forces. It is desirable that such plans have some built-in flexibility to accommodate unforeseen 
changes in work schedule, delays, or traffic patterns. 

The goal of any traffic control plan should be to safely route vehicle, bicycle, worker access, 
and pedestrian traffic, including persons with disabilities, through or around construction areas 
with geometries and traffic control devices as nearly comparable to those for normal operating 
situations as practical, while providing room for the contractor to work effectively. Policies for 
the use and application of signs and other traffic control devices when highway construction 
occurs are set forth in the MUTCD (6). It cannot be emphasized too strongly that the MUTCD (6) 
principles should be applied and a plan developed for the particular type of work performed. 

Adequate advance warning and sufficient follow-up information should be provided to 
drivers to prepare them for the changed operating conditions in construction areas. The distance 
that such signing should be located in advance of the work zone varies with the speed on the 
affected facility. Size of signs may vary depending on the need for greater legibility and emphasis 

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AASHTO — Geometric Design of Highways and Streets 



or the type of highway. Construction operations frequently create the need for adjustments in 
traffic patterns including the shifting of lanes. The minimum taper length for lane transitions in 
construction areas can be computed by a formula found in the MUTCD (6). Various 
configurations are illustrated in the MUTCD (6) and should be used in developing traffic control 
plans. 

The stopping of traffic by a flagger or any other means should be avoided wherever 
practical. Designs that provide for constant movement around an obstruction in the roadway, even 
if it is slow, are more acceptable and are less irritating to drivers than designs that require them to 
stop. 

When construction operations are scheduled to take place adjacent to passing traffic, a clear 
zone should be included in the traffic control plans, wherever practical, between the work space 
and the passing traffic. Under certain conditions, a positive barrier is justified. 

Traffic operational considerations for the design of a detour are speed, capacity, travel 
distance, and safety. The speed for a detour may be less than that on the facility being improved 
but should be high enough so as not to affect the capacity. When an existing highway or street is 
used as a detour, higher volumes result and it may be appropriate to increase the capacity of such 
a route in advance. The capacity is generally increased by eliminating troublesome turning 
movements, rerouting transit vehicles and trucks, banning parking, adopting and enforcing a 
loading/unloading ban during peak hours, eliminating or adjusting certain transit stops, 
coordinating signal timing, and sometimes physically widening the traveled way. An effective 
means of increasing capacity is by instituting a one-way detour system, coupled with parking 
restrictions. A detour plan is tested by comparing the traffic volumes expected to use the 
rearranged plan to the calculated capacity of the detour system. 

The roadway near construction access points should be well lighted and delineated. 
Channelization of traffic should be accomplished by the use of signing on yielding supports, 
pavement markings, and barricades. 

Construction areas, detours, and temporary connections often include geometric features and 
roadway environments that may need more caution and alertness than is normally expected of 
drivers. Care in the layout of these areas, in the use of delineation and warning devices, and in the 
establishment of areas for contractor operations is appropriate to minimize the impact on the 
safety of both motorists and workers. Items that should be considered in developing traffic 
control plans include the following: 

® Diversion and detour alignments to allow traffic to pass smoothly around the work 
zones. The surface of the traveled way, whether located within the construction area or 
on a detour, should be maintained in a condition that will permit the safe movement of 
traffic at a reasonable speed. 

® Adequate tapers for lane drops or where traffic is shifted laterally. Appropriate values 
for taper lengths can be found in the MUTCD (6). 



302 



Elements of Design 



In urban areas, diversion provisions for all existing pedestrian flows. The selected 

diversion paths should include safe roadway crossings, a smooth surface, and adequate 

width to accommodate persons with disabilities. 

Adequate traffic control devices and pavement markings for both daytime and 

nighttime effectiveness, including specifying temporary marking materials that can be 

removed when traffic-lane patterns change. 

Roadway illumination and warning lights where justified. Steady burning lights are 

used to delineate a continuous travel path through or around a work zone. The very 

short "on" time of flashing lights does not enable motorists to focus on the light and 

make a depth perception estimate. The use of flashers should be limited to marking a 

single object or condition, marking the start of a section using steady bum hghts, and 

for use with traffic control signs. 

The location of cones, delineators, drums, barriers, or barricades, to channelized traffic, 

when special conditions exist or if not shown in the standard plans. 

Policies concerning the removal of signs and markings from the job site, when they are 

no longer needed, if not provided for in the specifications. 

Except in extenuating circumstances, the removal of contractor equipment completely 

off the roadways, medians, and shoulders at night, on weekends, and whenever 

equipment is not in operation. In those instances where such removal is not practical, 

appropriate signing, lighting, barricades, barriers, and similar devices to protect the 

motorist from collision with the equipment should be specified. The storage of 

hazardous materials, however, should not be permitted on roadways, medians, or 

shoulders near the flow of traffic. 

A requirement in the plans or specifications for controlling or prohibiting the parking of 

employee's private vehicles in those areas on the project that may compromise the 

safety of workers and through traffic. 



REFERENCES 

1. Johansson, G., and K. Rumar. "Drivers' Brake Reaction Times," Human Factors, Vol. 13, 
No. 1, February 1971:23-27. 

2. Massachusetts Institute of Technology. Report of the Massachusetts Highway Accident 
Survey, CWA and ERA project, Cambridge, Mass.: Massachusetts Institute of Technology, 
1935. 

3. Normann, O. K. "Braking Distances of Vehicles from High Speeds," Proceedings HRB, Vol. 
22, Highway Research Board, 1953: 421-436. 

4. Fambro, D. B., K. Fitzpatrick, and R. J. Koppa. Determination of Stopping Sight Distances, 
NCHRP Report 400, Washington, D.C.: Transportation Research Board, 1997. 

5. AASHTO. Guidelines for Skid Resistant Pavement Design, Washington, D.C.: AASHTO, 
1976. 

6. U.S. Department of Transportation, Federal Highway Administration. Manual on Uniform 
Traffic Control Devices for Streets and Highways, Washington, D.C.: 1988 or most cuirent 
edifion. 

7. Alexander, G. J., and H. Lunenfeld. Positive Guidance in Traffic Control, Washington, D.C.: 
U.S. Department of Transportation, Federal Highway Administration, 1975. 

303 



AASHTO — Geometric Design of Highways and Streets 



8. King, G. F., and H. Lunenfeld. Development of Information Requirements and Transmission 
Techniques for Highway Users, NCHRP Report 123, Washington, D.C.: Transportation 
Research Board, 1971. 

9. McGee, H. W., W. Moore, B. G. Knapp, and J. H. Sanders. Decision Sight Distance for 
Highway Design and Traffic Control Requirements, Report No. FHWA-RD-78-78, McLean, 
Virginia: U.S. Department of Transportation, Federal Highway Administration, February 
1978. 

10. Robinson, G. H., D. J. Erickson, G. L. Thurston, and R. L. Clark. "Visual Search by 
Automobile Drivers," Human Factors, Vol 14, No. 4, August 1972: 315-323. 

11. Prisk, C. W. "Passing Practices on Rural Highways," Proceedings HRB, Vol. 21, Highway 
Research Board, 1941: 366-378. 

12. Weaver, G. D., and J. C Glennon. Passing Performance Measurements Related to Sight 
Distance Design, Report 134-6, College Station, Texas: Texas Transportation Institute, Texas 
A&M University, July 1971. 

13. Weaver, G. D., and D. L. Woods. Passing and No-Passing Signs, Markings, and Warrants, 
Report No. FHWA-RD-79-5, Washington, D.C.: U.S. Department of Transportation, Federal 
Highway Administration, September 1978. 

14. Transportation Research Board. Highway Capacity Manual, Special Report 209, Washington, 
D.C.: Transportation Research Board, 2000 or most current edition. 

15. Harwood, D. W., J. M. Mason, R. E. Brydia, M. T. Pietrucha, and G. L. Gittings. 
Intersection Sight Distance, NCHRP Report 383, Washington, D.C.: Transportation Research 
Board, 1996. 

16. Moyer, R. A. "Skidding Characteristics of Automobile Tires on Roadway Surfaces and Their 
Relation to Highway Safety," Bulletin No. 120, Ames, Iowa: Iowa Engineering Experiment 
Station, 1934, 

17. Stonex, K. A., and C. M. Noble. "Curve Design and Tests on the Pennsylvania Turnpike," 
Proceedings HRB, Vol. 20, Highway Research Board, 1940: 429-451. 

18. Moyer, R. A., and D. S. Berry. "Marking Highway Curves with Safe Speed Indications." 
Proceedings HRB, Vol. 20, Highway Research Board, 1940: 399-428. 

19. Bamett, J. "Safe Side Friction Factors and Superelevation Design," Proceedings HRB, Vol. 
16, Highway Research Board, 1936: 69-80. 

20. Bonneson, J. A. Superelevation Distribution Methods and Transition Designs, NCHRP 
Project 439, Washington, D.C.: Transportation Research Board, 2000. 

21. Hajela, G. P. Compiler, Resume of Tests on Passenger Cars on Winter Driving Surfaces, 
1939-1966, Chicago: National Safety Council, Committee on Winter Driving Hazards, 1968. 

22. MacAdam, C. C, P. S. Fancher, and L. Segal. Side Friction for Superelevation on Horizontal 
Curves, Report No. FHWA-RD-86-024, McLean, Virginia: U.S. Department of 
Transportation, Federal Highway Administration, August 1985. 

23. Tunnard, C, and B. Pushkarev. Man Made America: Chaos or Control? New Haven: Yale 
University Press, 1963. 

24. Bamett, J. Transition Curves for Highways, Washington, D.C.: Federal Works Agency, 
Public Roads Administration, 1940. 

25. Shortt, W. H. "A Practical Method for Improvement of Existing Railroad Curves," 
Proceedings Institution of Civil Engineering, Vol. 76, London: Institution of Civil 
Engineering, 1909: 97-208. 



304 



Elements of Design 



26. Bureau of Public Roads. Study of Speed Curvature Relations of Pentagon Road Network 
Ramps, unpublished data, Washington, D.C.: Federal Works Agency, Public Roads 
Administration, 1954. 

27. Cysewski, G. R. "Urban Intersectional Right Turning Movements," Traffic Engineering, 
Vol. 20, No. 1, October 1949: 22-37. 

28. George, L. E. "Characteristics of Left-Turning Passenger Vehicles," Proceedings HRB, 
Vol. 31, Highway Research Board, 1952: 374-385. 

29. Harwood, D. W., J. M. Mason, W. D. Glauz, B. T. Kulakowski, and K. Fitzpatrick. Truck 
Characteristics for Use in Highway Design and Operation, Report No. FHWA-RD-89-226, 
McLean, Virginia: U.S. Department of Transportation, Federal Highway Administration, 
August 1990. 

30. Offir aching Characteristics of Trucks and Truck Combinations, Research Committee Report 
No. 3, San Francisco, Cahfomia: Western Highway Institute, February 1970. 

31. AASHTO. Standard Specifications for Highway Bridges, Washington, D.C.: AASHTO, 
1996. 

32. Raymond, Jr., W. L. "Offsets to Sight Obstructions Near the Ends of Horizontal Curves," 
Civil Engineering, ASCE, Vol. 42, No. 1, January 1972: 71-72. 

33. Taragin, A. "Effect of Length of Grade on Speed of Motor Vehicles," Proceedings HRB, 
Vol. 25, Highway Research Board, 1945: 342-353. 

34. Willey, W. E. "Survey of Uphill Speeds of Trucks on Mountain Grades," Proceedings HRB, 
Vol. 29, Highway Research Board, 1949: 304-310. 

35. Huff, T. S., and F. H. Scrivner. "Simplified Climbing-Lane Design Theory and Road-Test 
Results," Bulletin 104, Highway Research Board, 1955: 1-11. 

36. Schwender, H. C, 0. K. Normann, and J. O. Granum. "New Method of Capacity 
Determination for Rural Roads in Mountainous Terrain," Bulletin 167, Highway Research 
Board, 1957: 10-37. 

37. Hayhoe, G. F., and J. G. Grundmann. Review of Vehicle Weight/Horsepower Ratio as Related 
to Passing Lane Design Criteria, Final Report of NCHRP Project 20-7(10), University Park, 
Pennsylvania: Pennsylvania State University, October 1978. 

38. Gillespie, T. Methods for Predicting Truck Speed Loss on Grades, Report No. FHWA/RD- 
86/059, McLean, Virginia: U.S. Department of Transportation, Federal Highway 
Administration, October 1986. 

39. Fancher, Jr., P. S., and T. D. Gillespie. Truck Operating Characteristics, NCHRP Synthesis 
of Highway Practice 241, Washington, D.C.: Transportation Research Board, 1997. 

40. Walton, C M., and C. E. Lee. Speed of Vehicles on Grades, Research Report 20-lF, Austin, 
Texas: Center for Highway Research, University of Texas at Austin, August 1975. 

41. Glennon, J. C. "An Evaluation of Design Criteria for Operating Trucks Safely on Grades," 
Highway Research Record 312, Highway Research Board, 1970: 93-1 12. 

42. Harwood, D. W., and C. J. Hoban. Low Cost Methods for Improving Traffic Operations on 
Two-Lane Roads, Report No. FHWA-IP-87-2, McLean, Virginia: U.S. Department of 
Transportation, Federal Highway Administration, 1987. 

43. Harwood, D. W., and A. D. St. John. Passing Lanes and Other Operational Improvements on 
Two-Lane Highways, Report No. FHWA/RD-85/028, McLean, Virginia: Federal Highway 
Administration, December 1985. 

44. Witheford, D. K. Truck Escape Ramps, NCHRP Synthesis of Highway Practice 178, 
Washington, D.C.: Transportation Research Board, May 1992. 

305 



AASHTO — Geometric Design of Highways and Streets 



45. Grade Severity Rating System Users Manual, Report No. FHWA-IP-88-015, McLean, 
Virginia: Federal Highway Administration, August 1989. 

46. Institute of Transportation Engineers. Truck Escape Ramps, Recommended Practice, 
Washington, D.C.: Institute of Transportation Engineers, 1989. 

47. Cron, F. W. 'The Art of Fitting the Highway to the Landscape," in W. B. Snow, ed., The 
Highway and the Landscape, New Brunswick, New Jersey: Rutgers University Press, 1959. 

48. Leisch, J. E. Application of Human Factors in Highway Design, unpubHshed paper presented 
at AASHTO Region 2 meeting, June 1975. 

49. AASHTO. A Guide for Transportation Landscape and Environmental Design, Washington, 
D.C.: AASHTO, 1991. 

50. Smith, B. L., and Lamm, Ruediger. "Coordination of Horizontal and Vertical Alignment with 
Regard to Highway Aesthetics," Transportation Research Record 1445, Transportation 
Research Board, 1994. 

51. ''Roads," Chapter Four in National Forest Landscape Management, Vol. 2, Forest Service, 
U.S. Department of Agriculture, March 1977. 

52. Practical Highway Aesthetics. New York, New York: ASCE, 1977. 

53. Design Guidelines for the Control of Blowing and Drifting Snow, Strategic Highway 
Research Program, National Research Council, 1994. 

54. AASHTO. Highway Drainage Guidelines, Vols. 1-11, Washington, D.C.: AASHTO, 1993. 

55. AASHTO. Model Drainage Manual, Washington, D.C.: AASHTO, 1991. 

56. Federal Highway Administration computer software and related publications are available 
from McTRANS, 512 Weil Hall, University of Florida, Gainesville, Florida 32611-2083. 
Phone (904) 392-0378 or PC-TRANS, 2011 Learned Hall, University of Kansas, Lawrence, 
Kansas, 66045. Telephone (913) 864-3199: 

HY 7. Bridge Waterways Analysis Model, (WSPRO), 1998. WSPRO Research Report. 
FHWA-RD-86-108, NTIS PB87-216107, WSPRO Users Manual (Version P60188), 

1990. FHWA-IP-89-27, NTIS PB218420. 
HY 8. FHWA Culvert Analysis (Version 6.1), 1999. Research Report (Version 1.0), 1987. 
HY 8 Applications Guide, 1987, FHWA-ED-87-101. 
HY 22, Urban Drainage Design Programs, Version 2.1, 1998. 
HYDRAIN. Drainage Design System (Version 6.1), 1999. 
HYDRAIN Users Manual, 1999. 

57. Richardson, E. V., et al. Highways in the River Environment: Hydraulic and Environmental 
Design Considerations, prepared by the Civil Engineering Department, Engineering Research 
Center, Colorado State University for the U.S. Department of Transportation, Federal 
Highway Administration. Washington, D.C.: February 1990. 

58. Federal Highway Administration Publications, Hydraulic Design Series (HDS) and Hydraulic 
Engineering Circulars (HEC). Washington, D.C.: U.S. Department of Transportation. 
Available from National Technical Information Service (NTIS), 5285 Port Royal Road, 
Springfield, VA 22161. Telephone (703) 487-4650: 

HDS 1. Hydraulics of Bridge Waterways, 1978. FHWA-EPD-86-101. NTIS PB86- 

181708. 
HDS 2. Highway Hydrology (SI), 1996. FHWA-SA-96-067, NTIS PB97-134290. 
HDS 3. Design Charts for Open-Channel Flow, 1961. FHWA-EPD-86-102. NTIS PB86- 

179249. 
HDS 4. Introduction to Hydraulics (SI), 1997. FHWA-HI-97-028. NTIS PB97-186761. 



306 



Elements of Design 



HDS 5. Hydraulic Design of Highway Culverts, 1985. FHWA-IP-65-15. NTIS PB86- 

196961. 
HEC 9. Debris-Control Structures, 1971. FHWA-EPD-86-106. NTIS PB86-179801. 
HEC 11. Design of Riprap Revetments, 1989, FHWA-lP-89-0106. NTIS PB89-218424. 
HEC 12. Drainage of Highway Pavements, 1984. FHWA-TS-84-202. NTIS PB84- 

215003. 
HEC 14. Hydraulic Design of Energy Dissipaters for Culverts and Channels, 1983. 

FHWA-EPD-86-110. NTIS PB86-180205. 
HEC 15. Design of Roadside Channels with Flexible Linings, 1988. FHWA-IP-87-7. 

NTIS PB89-122584. 
HEC 17. Design of Encroachments on Flood Plains Using Risk Analysis, 1981. FHWA 

EPD86-112. NTIS PB86-182110. 
HEC 18. Evaluating Scour at Bridges, 1995. FHWA-HI-96-301. NTIS PB96-163498. 
HEC 20. Stream Stability at Highway Structures, Edition 2, (SI), 1995. FHWA-HI-96- 

032. NTIS PB96-163480. 
HEC 21. Bridge Deck Drainage Systems, 1993. FHWA-SA-92-010. HTIS PB94-109584. 
HEC 22. Urban Drainage Design Manual (SI), 1996. FHWA-SA-96-078. NTIS PB97- 

199491. 
HEC 23. Bridge Scour and Stream Instability Countermeasures (SI), 1997. FHWA-HI- 

97-030. NTIS PB97-199491. 

59. AASHTO. A Guide for Development of Rest Areas on Major Arterials and Freeways, 
Washington, D.C.: AASHTO, 2001. 

60. AASHTO. An Informational Guide for Roadway Lighting, Washington, D.C.: AASHTO, 
1984. 

61. AASHTO. A Policy on the Accommodation of Utilities Within Freeway Right-of-Way, 
Washington, D.C.: AASHTO, 1989. 

62. AASHTO. A Guide for Accommodating Utilities Within Highway Right-of-Way, Washington, 
D.C.: AASHTO, 1994. 

63. AASHTO. Roadside Design Guide, Washington, D.C.: AASHTO, 1996. 

64. AASHTO. Standard Specifications for Structural Supports for Highway Signs, Luminaires, 
and Traffic Signals, Washington, D.C.: AASHTO, 1994. 

65. Brudis and Associates, Inc. Advisory Speeds on Maryland Roads, Hanover, Maryland: 
Maryland Department of Transportation, Office of Traffic and Safety, August 1999. 



307 



CHAPTER 4 
CROSS SECTION ELEMENTS 

GENERAL 

To assure consistency in this policy, the terms "roadway" and "traveled way" are defined by 
AASHTO as follows: 

Roadway: The portion of a highway, including shoulders, for vehicular use. A divided 
highway has two or more roadways (see Exhibits 4-1 and 4-2). 

Traveled way: The portion of the roadway for the movement of vehicles, exclusive of 
shoulders (see Exhibits 4-1 and 4-2). 



PAVEMENT 
Surface Type 

The selection of pavement type is determined based on the traffic volume and composition, 
soil characteristics, weather, performance of pavements in the area, availability of materials, 
energy conservation, initial cost, and the overall annual maintenance and service-life cost. The 
structural design of pavements is not included in this policy, but is addressed in the AASHTO 
Guide for Design of Pavement Structures (1). 

Important pavement characteristics that are related to geometric design are the effect on 
driver behavior and the ability of a surface to retain its shape and dimensions, to drain, and to 
retain adequate skid resistance. High-type pavements retain their shape and do not ravel at the 
edges if placed on a stable subgrade. Their smoothness and proper cross-slope design enable 
drivers to steer easily and keep their vehicles moving in the proper path. At the other extreme, 
low-type surfaces have a tendency toward raveling, which reduces their effective width and 
requires greater steering effort to maintain a correct path. Accordingly, low-type surfaces are used 
where traffic volume is light. 

While the selection of design speed is dependent on many factors other than pavement 
surface type, high-type surfaces provide for higher operafing speeds than do low-type surfaces. 
Therefore, the surface type provided should be consistent with the selected design speed for the 
highway. 



Cross Slope 

Undivided traveled ways on tangents, or on flat curves, have a crown or high point in the 
middle and a cross slope downward toward both edges. Unidirectional cross slopes across the 



309 



AASHTO — Geometric Design of Highways and Streets 



^ Rounding 







Drolnaga 



Sl<j#wdlk 




Byidlng Line 




NOTE: 



TW = Traveled Way 
S = Usable Shoulder 
• ^- Rate of Slope 2 to 6 PerGent 



Exhibit 4"1» Typical Cross Section, Normal Crown 



3]0 



Cross Section Elements 



"Rol!-Over% kSqmro\c Off f erenc© 
In Rett Of €rom Slept # Not to 

Excs@d 8 % 



Vorlobia 




Rounding Alternate 



Roadwoy 



Vorlobla 




Roadway 



VorlciDia 




NOTEi 



S " UaoWa Shoulder 

# " Suparelavctfon Hofm C©l there Gr@otar 
Thon Nor mol Shoulder Slop® 



Exhibit 4"2» Typical Cross Sectio^j Superelevated 



311 



AASHTO — Geometric Design of Highways and Streets 



entire width of the traveled way may be utiUzed. The downward cross slope may be a plane or 
rounded section or a combination. With plane cross slopes, there is a cross slope break at the 
crown line and a uniform slope on each side. Rounded cross sections usually are parabolic, with a 
slightly rounded surface at the crown line and increasing cross slope toward the edge of the 
traveled way. Because the rate of cross slope is variable, the parabolic section is described by the 
crown height (i.e., the vertical drop from the center crown line to the edge of the traveled way). 
The rounded section is advantageous in that the cross slope steepens toward the edge of the 
traveled way, thereby facilitating drainage. Disadvantages are that rounded sections are more 
difficult to construct, the cross slope of the outer lanes may be excessive, and warping of 
pavement areas at intersections may be awkward or difficult to construct. 

On divided highways each one-way traveled way may be crowned separately as on two-lane 
highways, or it may have a unidirectional cross slope across the entire width of the traveled way, 
which is almost always downward to the outer edge. A cross section with each roadway crowned 
separately, as shown in Exhibit 4-3A through Exhibit 4-3C, has an advantage in rapidly draining 
the pavement during rainstorms. In addition, the difference between high and low points in the 
cross section is minimal. Disadvantages are that more inlets and underground drainage lines are 
needed, and treatment of intersections is more difficult because of the number of high and low 
points on the cross section. Use of such sections should preferably be limited to regions of high 
rainfall or where snow and ice are major factors. Sections having no curbs and a wide depressed 
median are particularly well-suited for these conditions. 










^^^: 



iisfeiinzriirzxzr::^^^^ 



• c- 



EACH PAVEMENT SLOPES TWO WAYS 





EACH PAVEMENT SLOPES ONE HAY 



Exhibit 4-3. Roadway Sections for Divided Highway (Basic Cross Slope Arrangements) 



312 



Cross Section Elements 



Roadways with unidirectional cross slopes, as shown in Exhibit 4-3D through Exhibit 4-3G, 
tend to provide more comfort to drivers when they change lanes and may either drain away from 
or toward the median. Drainage away from the median may effect a savings in drainage 
structures, minimize drainage across the inner, higher-speed lanes, and simplify treatment of 
intersecting streets. Drainage toward the median is advantageous in that the outer lanes, which are 
used by most traffic, are more free of surface water. This surface runoff, however, should then be 
collected into a single conduit under the median. Where curbed medians exist, drainage is 
concentrated next to or on higher-speed lanes. When the median is narrow, this concentration 
results in splashing on the windshields of opposing traffic. 

The rate of cross-slope is an important element in cross-section design. Superelevation on 
curves is determined by the speed-curvature relationships given in Chapter 3, but cross slope or 
crown on tangents or on long-radius curves are complicated by two contradictory controls. On 
one hand, a reasonably steep lateral slope is desirable to minimize ponding of water on pavements 
with flat profile grades as a result of pavement imperfections or unequal settlement. A steep cross 
slope is also desirable on curbed pavements to confine water flow to a narrow width of pavement 
adjacent to the curb. On the other hand, steep cross slopes are undesirable on tangents because of 
the tendency of vehicles to drift toward the low edge of the traveled way. This drifting becomes a 
major concern in areas where snow and ice are common. Cross slopes up to and including 
2 percent are barely perceptible in terms of vehicle steering. However, cross slopes steeper than 
2 percent are noticeable and require a conscious effort in steering. Furthermore, steep cross slopes 
increase the susceptibility to lateral skidding when vehicles brake on icy or wet pavements or 
when stops are made on dry pavements under emergency conditions. 

The prevalence of high winds may significantly alter the effect of cross slope on steering. In 
rolling or mountainous terrain with alternate cut-and-fill sections or in areas alternately forested 
and cleared, any substantial cross wind produces an intermittent impact on a vehicle moving 
along the highway and affects its steering. In areas where such conditions are likely, it is desirable 
to avoid high rates of cross slope. 

On high-type two-lane roadways, crowned at the center, the accepted rate of cross slope 
ranges from 1.5 to 2 percent. When three or more lanes are inclined in the same direction on 
multilane highways, each successive pair of lanes or portion thereof outward from the first two 
lanes from the crown line may have an increased slope. The two lanes adjacent to the crown line 
should be pitched at the normal minimum slope, and on each successive pair of lanes or portion 
thereof outward, the rate may be increased by about 0.5 to 1 percent. As shown in Exhibit 4-3G, 
the left side has a continuous sloped pavement while the right has an increased slope on the outer 
lane. 

Use of cross slopes steeper than 2 percent on high-type, high-speed highways with a central 
crown line is not desirable. In passing maneuvers, drivers cross and recross the crown line and 
negotiate a total rollover or cross-slope change of over 4 percent. The reverse curve path of travel 
of the passing vehicle causes a reversal in the direction of centrifugal force, which is further 
exaggerated by the effect of the reversing cross slopes. Trucks with high centers of gravity 
crossing over the crown line are caused to sway from side to side when traveling at high speed, at 



313 



AASHTO — Geometric Design of Highways and Streets 



which time control may be difficult to maintain. Exhibits 4-3A through 4-3C are examples of 
roadway conditions where this situation would be encountered. 

In areas of intense rainfall, a somewhat steeper cross slope may be needed to facilitate 
roadway drainage. In such cases, the cross slope on high-type pavements may be increased to 
2.5 percent, with a coixesponding crown line crossover of 5 percent. Where three or more lanes 
are provided in each direction, the maximum cross slope should be limited to 4 percent. Use of 
this increased cross slope should be limited to the condition described in the preceding 
discussion. For all other conditions, a maximum cross slope of 2 percent should be used for high- 
type pavements. In locations of intense rainfall and where the maximum cross slope is used, 
consideration should be given to the use of grooving or open-graded mixes. 

The cross slope rates discussed above pertain largely to high-type surfaces. A greater cross 
slope should be utilized for low-type surfaces. Exhibit 4-4 shows a range of values applicable to 
each type of surface. 



Surfae© type Range iri cross-slQpe rate (%) 



High 1.5-2 

Low 2-6 



Exhibit 4-4. Normal Traveled- Way Cross Slope 



Because of the nature of the surfacing materials used and surface irregularities, low-type 
surfaces such as earth, gravel, or crushed stone need an even greater cross slope on tangents to 
prevent the absorption of water into the surface. Therefore, cross slopes greater than 2 percent 
may be used on these types of surfaces. 

Where roadways are designed with outer curbs, the lower values in the ranges of cross 
slopes in Exhibit 4-4 are not recommended because of the increased likelihood of there being a 
sheet of water over a substantial part of the traveled way adjacent to the curb. For any rate of 
rainfall, the width of traveled way that is inundated with water varies with the rate of cross slope, 
roughness of gutter, frequency of discharge points, and longitudinal grade. A cross slope greater 
than 1 percent is desirable, and in some cases, a cross slope of more than 1.5 percent is needed to 
limit inundation to about half of the outer traffic lane. A cross slope of 1.5 percent is suggested as 
a practical minimum for curbed high-type pavement. Curbs with steeper adjacent gutter sections 
may permit the use of lesser rates of cross slope. A preferred cross-section treatment is the use of 
a straight shoulder slope and the avoidance of curbs, whenever practical. 



Skid Resistance 

Skidding crashes are a major concern in highway safety. It is not sufficient to attribute 
skidding crashes merely to "driver error" or "driving too fast for existing conditions." The 



314 



Cross Section Elements 



roadway should provide a level of skid resistance that will accommodate the braking and steering 
maneuvers that can reasonably be expected for the particular site. 

Research has demonstrated that highway geometries affect skidding (2). Therefore, skid 
resistance should be a consideration in the design of all new construction and major 
reconstruction projects. Vertical and horizontal alignments can be designed in such a way that the 
potential for skidding is reduced. Also, improvements to the vertical and horizontal alignments 
should be considered as a part of any reconstruction project. 

Pavement types and textures also affect a roadway's skid resistance. The four main causes of 
poor skid resistance on wet pavements are rutting, polishing, bleeding, and dirty pavements. 
Rutting causes water accumulation in the wheel tracks. Polishing reduces the pavement surface 
microtexture and bleeding can cover it. In both cases, the harsh surface features needed for 
penetrating the thin water film are diminished. Pavement surfaces will lose their skid resistance 
when contaminated by oil drippings, layers of dust, or organic matter. Measures taken to correct 
or improve skid resistance should result in the following characteristics: high initial skid 
resistance durability, the ability to retain skid resistance with time and traffic, and minimum 
decrease in skid resistance with increasing speed. 

Tining during placement leaves indentations in the pavement surface and has proved to be 
effective in reducing the potential for hydroplaning on roadways with portland cement concrete 
surfaces. The use of surface courses or overlays constructed with polish-resistant coarse 
aggregate is the most widespread method for improving the surface texture of bituminous 
pavements. Overlays of open-graded asphalt friction courses are quite effective because of their 
frictional and hydraulic properties. For further discussion, refer to the AASHTO Guidelines for 
Skid Resistant Pavement Design (3). 



LANE WIDTHS 

The lane width of a roadway greatly influences the safety and comfort of driving. Lane 
widths of 2.7 to 3.6 m [9 to 12 ft] are generally used, with a 3.6-m [12-ft] lane predominant on 
most high-type highways. The extra cost of providing a 3.6-m [12-ft] lane width, over the cost of 
providing a 3.0-m [10-ft] lane width is offset to some extent by a reduction in cost of shoulder 
maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the 
pavement edges. The wider 3.6-m [12-ft] lane provides desirable clearances between large 
commercial vehicles traveling in opposite directions on two-lane, two-way rural highways when 
high traffic volumes and particularly high percentages of commercial vehicles are expected. 

Lane widths also affect highway level of service. Narrow lanes force drivers to operate their 
vehicles closer to each other laterally than they would normally desire. Restricted clearances have 
much the same effect. In a capacity sense the effective width of traveled way is reduced when 
adjacent obstructions such as retaining walls, bridge trusses or headwalls, and parked cars restrict 
the lateral clearance. Further information on the effect of lane width on capacity and level of 
service is presented in the Highway Capacity Manual (HCM) (4). In addition to the capacity 
effect, the resultant erratic operation has an undesirable effect on driver comfort and crash rates. 

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AASHTO — Geometric Design of Highways and Streets 



Where unequal -width lanes are used, locating the wider lane on the outside (right) provides 
more space for large vehicles that usually occupy that lane, provides more space for bicycles, and 
allows drivers to keep their vehicles at a greater distance from the right edge. Where a curb is 
used adjacent to only one edge, the wider lane should be placed adjacent to that curb. The basic 
design decision is the total roadway width, while the placement of stripes actually determines the 
lane widths. 

Although lane widths of 3.6 m [12 ft] are desirable on both rural and urban facilities, there 
are circumstances where lanes less than 3.6 m [12 ft] wide should be used. In urban areas where 
pedestrian crossings, right-of-way, or existing development become stringent controls, the use of 
3.3-m [11-ft] lanes is acceptable. Lanes 3.0 m [10 ft] wide are acceptable on low-speed facilities, 
and lanes 2.7 m [9 ft] wide are appropriate on low-volume roads in rural and residential areas. For 
further information, see NCHRP Report 362, Roadway Widths for Low-Traffic Volume Roads (5). 
In some instances, on multilane facilities in urban areas, narrower inside lanes may be utilized to 
permit wider outside lanes for bicycle use. In this situation, 3.0-m to 3.3-m [10- to 1 1-ft] lanes are 
common on inside lanes with 3.6-m to 3.9-m [12- to 13-ft] lanes utilized on outside lanes. 

Auxiliary lanes at intersections and interchanges often help to facilitate traffic movements. 
Such added lanes should be as wide as the through-traffic lanes but not less than 3.0 m [10 ft]. 
Where continuous two-way left-turn lanes are provided, a lane width of 3.0 m to 4.8 m [10 to 
16 ft] provides the optimum design. 

It may not be cost-effective to design the lane and shoulder widths of local and collector 
roads and streets that carry less than 400 vehicles per day using the same criteria applicable to 
higher volume roads or to make extensive operational and safety improvements to such very low- 
volume roads. AASHTO is currently evaluating alternative design criteria for local and collector 
roads and streets that carry less than 400 vehicles per day based on a safety risk assessment. 



SHOULDERS 
General Characteristics 

A shoulder is the portion of the roadway contiguous with the traveled way that 
accommodates stopped vehicles, emergency use, and lateral support of subbase, base, and surface 
courses. In some cases, the shoulder can accommodate bicyclists. It varies in width from only 
0.6 m [2 ft] on minor rural roads where there is no surfacing, or the surfacing is applied over the 
entire roadbed, to approximately 3.6 m [12 ft] on major roads where the entire shoulder may be 
stabilized or paved. 

The term "shoulder" is variously used with a modifying adjective to describe certain 
functional or physical characteristics. The following meanings apply to the terms used here: 



316 



Cross Section Elements 



The "graded" width of shoulder is that measured from the edge of the traveled way to 
the intersection of the shoulder slope and the foreslope planes, as shown in 
Exhibit 4-5A. 

The "usable" width of shoulder is the actual width that can be used when a driver makes 
an emergency or parking stop. Where the sideslope is 1V:4H or flatter, the "usable" 
width is the same as the "graded" width since the usual rounding 1.2 to 1.8 m [4 to 6 ft] 
wide at the shoulder break will not lessen its useful width appreciably. Exhibits 4-5B 
and 4-5C illustrate the usable shoulder width. 




Exhibit 4-5. Graded and Usable Shoulders 



Shoulders may be surfaced either full or partial width to provide a better all-weather load 
support than that afforded by native soils. Materials used to surface shoulders include gravel, 
shell, crushed rock, mineral or chemical additives, bituminous surface treatments, and various 
forms of asphaltic or concrete pavements. 

The shoulder on minor rural roads with low traffic volume serves essentially as structural 
lateral support for the surfacing and as an additional width for the traveled way. This permits 
drivers meeting or passing other vehicles to drive on the edge of the roadway without leaving the 
surfacing, thus making use of the shoulder itself. Roads with a narrow traveled way, narrow 
shoulders, and an appreciable traffic volume tend to provide poor service, have a relatively higher 
crash rate, and need frequent and costly maintenance. 



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AASHTO — Geometric Design of Highways and Streets 



Well-designed and properly maintained shoulders are needed on rural highways with an 
appreciable volume of traffic, on freeways, and on some types of urban highways. Their 
advantages include: 

© Space is provided away from the traveled way for vehicles to stop because of 

mechanical difficulties, flat tires, or other emergencies. 
® Space is provided for motorists to stop occasionally to consult road maps or for other 

reasons. 
® Space is provided for evasive maneuvers to avoid potential crashes or reduce their 

severity. 
® The sense of openness created by shoulders of adequate width contributes to driving 

ease and reduced stress. 
® Sight distance is improved in cut sections, thereby potentially improving safety. 
® Some types of shoulders enhance highway aesthetics. 
® Highway capacity is improved because uniform speed is encouraged. 
® Space is provided for maintenance operations such as snow removal and storage. 

Lateral clearance is provided for signs and guardrails. 
® Storm water can be discharged farther from the traveled way, and seepage adjacent to 

the traveled way can be minimized. This may directly reduce pavement breakup. 
® Structural support is given to the pavement. 
^ Space is provided for pedestrian and bicycle use, for bus stops, for occasional 

encroachment of vehicles, for mail delivery vehicles, and for the detouring of traffic 

during construction. 



® 



For further information on other uses of shoulders, refer to NCHRP Report 254, Shoulder 
Geometries and Use Guidelines (6). 

Urban highways generally have curbs along the outer lanes. A stalled vehicle, during peak 
hours, disturbs traffic flow in all lanes in that direction when the outer lane serves through-traffic. 
Where on-street parking is permitted, the parking lane provides some of the same services listed 
above for shoulders. Parking lanes are discussed later in this chapter in the section on "On-Street 
Parking." 



Width of Shoulders 

Desirably, a vehicle stopped on the shoulder should clear the edge of the traveled way by at 
least 0.3 m [T ft], and preferably by 0.6 m [2 ft]. This preference has led to the adoption of 3.0 m 
[10 ft] as the normal shoulder width that should be provided along high-type facilities. In difficult 
terrain and on low-volume highways, shoulders of this width may not be practical. A minimum 
shoulder width of 0.6 m [2 ft] should be considered for the lowest-type highway, and a 1.8- to 
2.4-m [6- to 8-ft] shoulder width is preferable. Heavily traveled, high-speed highways and 
highways carrying large numbers of trucks should have usable shoulders at least 3.0 m [10 ft] 
wide and preferably 3.6 m [12 ft] wide; however, widths greater than 3.0 m [10 ft] may encourage 
unauthorized use of the shoulder as a travel lane. Where bicyclists and pedestrians are to be 
accommodated on the shoulders, a minimum usable shoulder width (i.e., clear of rumble strips) of 

318 



Cross Section Elements 



1.2 m [4 ft] should be used. For additional information on shoulder widths to accommodate 
bicycles, see the AASHTO Guide for the Development of Bicycle Facilities (7). Shoulder widths 
for specific classes of highways are discussed in Chapters 5 through 8. 

Where roadside barriers, walls, or other vertical elements are present, it is desirable to 
provide a wide enough graded shoulder that the vertical elements will be offset a minimum of 
0.6 m [2 ft] from the outer edge of the usable shoulder. To provide lateral support for guardrail 
posts and/or clear space for lateral dynamic deflection of the particular barrier in use, it may be 
appropriate to provide a graded shoulder that is wider than the shoulder where no vertical 
elements are present. On low-volume roads, roadside barriers may be placed at the outer edge of 
the shoulder; however, a minimum clearance of 1.2 m [4 ft] should be provided from the traveled 
way to the barrier. 

Although it is desirable that a shoulder be wide enough for a vehicle to be driven completely 
off the traveled way, narrower shoulders are better than none at all. For example, when a vehicle 
making an emergency stop can pull over onto a nan*ow shoulder such that it occupies only 0.3 to 
1.2 m [1 to 4 ft] of the traveled way, the remaining traveled way width can be used by passing 
vehicles. Partial shoulders are sometimes used where full shoulders are unduly costly, such as on 
long (over 60 m [200 ft]) bridges or in mountainous terrain. 

Regardless of the width, a shoulder should be continuous. The full benefits of a shoulder are 
not realized unless it provides a driver with refuge at any point along the traveled way. A 
continuous shoulder provides a sense of security such that almost ail drivers making emergency 
stops will leave the traveled way. With intermittent sections of shoulder, however, some drivers 
will find it necessary to stop on the traveled way, creating an undesirable situation. A continuous 
paved shoulder provides an area for bicyclists to operate without obstructing faster moving motor 
vehicle traffic. Although continuous shoulders are preferable, narrow shoulders and intermittent 
shoulders are superior to no shoulders. Intermittent shoulders are briefly discussed below in the 
section on "Turnouts." 

Shoulders on structures should nonnally have the same width as usable shoulders on the 
approach roadways. As previously discussed, the narrowing or loss of shoulders, especially on 
structures, may cause serious operational and safety problems. Long, high-cost structures usually 
warrant detailed special studies to determine practical dimensions. Reduced shoulder widths may 
be considered in rare cases. A discussion of these conditions is provided in Chapters 7 and 10. 



Shoulder Cross Sections 

Important elements in the lateral drainage systems, shoulders should be flush with the 
roadway surface and abut the edge of the traveled way. All shoulders should be sloped to drain 
away from the traveled way on divided highways with a depressed median. With a raised narrow 
median, the median shoulders may slope in the same direction as the traveled way. However, in 
regions with snowfall, median shoulders should be sloped to drain away from the traveled way to 
avoid melting snow draining across travel lanes and refreezing. All shoulders should be sloped 
sufficiently to rapidly drain surface water, but not to the extent that vehicular use would be 

319 



AASHTO — Geometric Design of Highways and Streets 



restricted. Because the type of shoulder construction has a bearing on the cross slope, the two 
should be determined jointly. Bituminous and concrete-surfaced shoulders should be sloped from 
2 to 6 percent, gravel or crushed-rock shoulders from 4 to 6 percent, and turf shoulders from 6 to 
8 percent. Where curbs are used on the outside of shoulders, the cross slope should be 
appropriately designed with the drainage system to prevent ponding on the traveled way. 

It should be noted that rigid adherence to the slope rates outlined in this chapter may present 
minor traffic operational problems if they are applied without regard to the cross section of the 
paved surface. On tangent or long-radius curved alignment with normal crown and turf shoulders, 
the maximum algebraic difference in the traveled way and shoulder grades should be from 6 to 
7 percent. Although this maximum algebraic difference in slopes is not desirable, it is tolerable 
due to the benefits gained in pavement stability by avoiding storm water detention at the 
pavement edge. 

Shoulder slopes that drain away from the paved surface on the outside of well-superelevated 
sections should be designed to avoid too great a cross-slope break. For example, use of a 
4 percent shoulder cross slope in a section with a traveled way superelevation of 8 percent results 
in a 12 percent algebraic difference in the traveled way and shoulder grades at the high edge-of- 
traveled way. Grade breaks of this order are not desirable and should not be permitted 
(Exhibit 4-2A). It is desirable that all or part of the shoulder should be sloped upward at about the 
same rate or at a lesser rate than the superelevated traveled way (see the dashed line labeled 
Alternate in Exhibit 4-2 A). Where this is not desirable because of storm water or melting snow 
and ice draining over the paved surface, a compromise might be used in which the grade break at 
the edge of the paved surface is limited to approximately 8 percent by flattening the shoulder on 
the outside of the curve (Exhibit 4-2B). 

One means of avoiding too severe of a grade break is the use of a continuously rounded 
shoulder cross section on the outside of the superelevated traveled way (Exhibit 4-2C). The 
shoulder in this case is a convex section continuing from the superelevation slope instead of a 
sharp grade break at the intersection of the shoulder and traveled way slopes. In this method, 
some surface water will drain upon the traveled way; however, this disadvantage is offset by the 
benefit of a smoother transition for vehicles that may accidentally or purposely drive upon the 
shoulder. It should also be noted that convex shoulders present more difficulties in construction 
than do planar sections. An alternate method to the convex shoulder consists of a planar shoulder 
section with multiple breaks in the cross slope. Shoulder cross slopes on the high side of a 
superelevated section that are substantially less than those discussed above are generally not 
detrimental to shoulder stability. There is no discharge of storm water from the traveled way to 
the shoulder and, therefore, little likelihood of shoulder erosion damage. 

In some areas, shoulders are designed with a curb or gutter at the outer edge to confine 
runoff to the paved shoulder area. Drainage for the entire roadway is handled by these curbs, with 
the runoff directed to selected outlets. The outer portion of the paved shoulder serves as the 
longitudinal gutter. Cross slopes should be the same as for shoulders without a curb or gutter, 
except that the slope may be increased somewhat on the outer portion of the shoulder. This type 
of shoulder is advantageous in that the curb on the outside of the shoulder does not deter 
motorists from driving off the traveled way, and the shoulder serves as a gutter in keeping storm 

320 



Cross Section Elements 



water off the traveled lanes. Proper delineation should adequately distinguish the shoulder from 
the traveled way. 



Shoulder Stability 

If shoulders are to function effectively, they should be sufficiently stable to support 
occasional vehicle loads in all kinds of weather without rutting. Evidence of rutting, skidding, or 
vehicles being mired down, even for a brief seasonal period, may discourage and prevent the 
shoulder from being used as intended. 

All types of shoulders should be constructed and maintained flush with the traveled way 
pavement if they are to fulfill their intended function. Regular maintenance is needed to provide a 
flush shoulder. Unstabilized shoulders generally undergo consolidation with time, and the 
elevation of the shoulder at the traveled way edge tends to become lower than the traveled way. 
The drop-off can adversely affect driver control when driving onto the shoulder at any 
appreciable speed. In addition, when there is no visible assurance of a flush stable shoulder, the 
operational advantage of drivers staying close to the pavement edge is reduced. 

Paved or stabilized shoulders offer numerous advantages, including: (1) provision of refuge 
for vehicles during emergency situations, (2) elimination of rutting and drop-off adjacent to the 
edge of the traveled way, (3) provision of adequate cross slope for drainage of roadway, (4) 
reduction of maintenance, and (5) provision of lateral support for roadway base and surface 
course. 

Shoulders with turf growth may be appropriate, under favorable climatic and soil conditions, 
for local roads and some collectors. Turf shoulders are subject to a buildup that may inhibit 
proper drainage of the traveled way unless adequate cross slope is provided. When wet, the turf 
may be slippery unless closely mowed and on granular soil. Turf shoulders offer good traveled- 
way delineation and do not invite use as a traffic lane. Stabilized turf shoulders need little 
maintenance other than mowing. 

Based on experience, drivers are wary of unstabilized shoulders, especially on high-volume 
highways, such as suburban expressways. Such experience has led to the replacement of 
unstabilized shoulders with some form of stabilized or surfaced shoulders. 

In some areas, rural highways are built with surfacing over the entire width, including 
shoulders. Depending upon the conditions, this surfacing may be from about 8.4 to 13.2 m 
[28 to 44 ft] wide for two-lane roads. This type of treatment protects shoulders from erosion and 
also protects the subgrade from moisture penetration, thereby enhancing the strength and 
durability of the pavement. Also, edge stripes are generally used to delineate the edge of the 
traveled way, but in some cases there is no indication of the edge of traveled way. This design is 
desirable because a continuous shoulder is provided, even if its separate width is not apparent. 

Experience on heavy-volume facilities shows that, on occasion, traffic will use smooth- 
surfaced shoulders as through-traffic lanes. On moderate-to-steep grades, trucks may pull to the 

321 



AASHTO — Geometric Design of Highways and Streets 



right and encroach upon the shoulder. While such shoulder encroachments are undesirable, this 
does not warrant the elimination of the surfaced shoulder because of factors such as high-volume 
traffic and truck usage. 



Shoulder Contrast 

It is desirable that the color and texture of shoulders be different from those of the traveled 
way. This contrast serves to clearly define the traveled way at all times, particularly at night and 
during inclement weather, while discouraging the use of shoulders as additional through lanes. 
Bituminous, crushed stone, gravel, and turf shoulders all offer excellent contrast with concrete 
pavements. Satisfactory contrast with bituminous pavements is more difficult to achieve. Various 
types of stone aggregates and turf offer good contrast. Several states have attempted to achieve 
contrast by seal-coating shoulders with lighter color stone chips. Unfortunately, the color 
distinction may diminish in a few years. The use of edge lines as described in the Manual on 
Uniform Traffic Control Devices (MUTCD) (8) reduces the need for shoulder contrast. Edge lines 
should be applied where shoulder use by bicycles is expected. Some states have provided 
depressed rumble strips in the shoulder to provide an audible alert to the motorists that they have 
crossed over onto the shoulder. This is particularly effective at night and during inclement 
weather. However, care should be used if the shoulders are to be used by bicyclists. 

Turnouts 

It is not always economically practical to provide wide shoulders continuously along the 
highway, especially where the alignment passes through deep rock cuts or where other conditions 
limit the cross-section width. In such cases, consideration should be given to the use of 
intermittent sections of shoulder or turnouts along the highway. Such turnouts provide an area for 
emergency stops and also allow slower moving vehicles to pull out of the through lane to permit 
following vehicles to pass. 

Proper design of turnouts should consider turnout length, including entry and exit tapers, 
turnout width, and the location of the turnout with respect to horizontal and vertical curves where 
sight distance is limited. Turnouts should be located so that approaching drivers have a clear view 
of the entire turnout in order to determine whether the turnout is available for use (9). Where 
bicycle traffic is expected, turnouts should be paved so bicyclists may move aside to allow faster 
traffic to pass. 



HORIZONTAL CLEARANCE TO OBSTRUCTIONS 

The term "clear zone" is used to designate the unobstructed, relatively flat area provided 
beyond the edge of the traveled way for the recovery of errant vehicles. The clear zone includes 
any shoulders or auxiliary lanes. 

The AASHTO Roadside Design Guide (10) discusses clear zone widths as related to speed, 
volume, and embankment slope. The Guide may be used as a reference for determination of 

322 



Cross Section Elements 



clear-zone widths for freeways, rural arterials, and high-speed rural collectors. For low-speed 
rural collectors and rural local roads, a minimum clear-zone width of 3.0 m [10 ft] should be 
provided. 

For urban arterials, collectors, and local streets where curbs are utilized, space for clear 
zones is generally restricted. A minimum offset distance of 500 mm [18 in] should be provided 
beyond the face of the curb, with wider offsets provided where practical. This "operational" offset 
will generally permit curbside parking and will not have a negative impact on traffic flow. 
However, since most curbs do not have a significant capability to redirect vehicles, a minimum 
clear zone distance commensurate with prevailing traffic volumes and vehicle speeds should be 
provided where practical. 

CURBS 

General Considerations 

The type and location of curbs affects driver behavior and, in turn, the safety and utility of a 
highway. Curbs serve any or all of the following purposes: drainage control, roadway edge 
delineation, right-of-way reduction, aesthetics, delineation of pedestrian walkways, reduction of 
maintenance operations, and assistance in orderly roadside development. A curb, by definition, 
incorporates some raised or vertical element. 

Curbs are used extensively on all types of low-speed urban highways, as defined in the 
Design Speed section in Chapter 2. In the interest of safety, caution should be exercised in the use 
of curbs on high-speed rural highways. Where curbs are needed along high-speed rural highways 
due to drainage considerations, the need for access control, restricted right-of-way, or other 
reasons, they should always be located at the outside edge of the shoulder. 

While cement concrete curbs are installed by some highway agencies, granite curbs are used 
where the local supply makes them economically competitive. Because of its durability, granite is 
preferred over cement concrete where deicing chemicals are used for snow and ice removal 

Conventional concrete or bituminous curbs offer little visible contrast to normal pavements, 
particularly during fog or at night when surfaces are wet. The visibility of channelizing islands 
with curbs and of continuous curbs along the edges of the traveled way may be improved through 
the use of reflectorized markers that are attached to the top of the curb. 

In another form of high-visibility treatment, reflectorized paints or other reflectorized 
surfaces, such as applied thermoplastic, can make curbs more conspicuous. However, to be kept 
fully effective, reflectorized curbs need periodic cleaning or repainting, which usually involves 
substantial maintenance costs. Curb markings should be placed in accordance with the 

MUTCD (8). 



323 



AASHTO — Geometric Design of Highways and Streets 



Curb Configurations 

Curb configurations include both vertical and sloping curbs. Exhibit 4-6 illustrates several 
curb configurations that are commonly used. A curb may be designed as a separate unit or 
integrally with the pavement. Vertical and sloping curb designs may include a gutter, forming a 
combination curb and gutter section. 

Vertical curbs may be either vertical or nearly vertical and are intended to discourage 
vehicles from leaving the roadway. As shown in Exhibit 4-6A, they range from 150 to 200 mm 
[6 to 8 in] in height. Vertical curbs should not be used along freeways or other high-speed 
roadways because an out-of-control vehicle may overturn or become airborne as a result of an 
impact with such a curb. Since curbs are not adequate to prevent a vehicle from leaving the 
roadway, a suitable traffic barrier should be provided where redirection of vehicles is needed. 

Vertical curbs and safety walks may be desirable along the faces of long walls and tunnels, 
particularly if full shoulders are not provided. These curbs tend to discourage vehicles from 
driving close to the wall, and thus the safety walk, reducing the risk to persons walking from 
disabled vehicles. 

Sloping curbs are designed so vehicles can cross them readily when the need arises. As 
shown in Exhibit 4-6B through 4-6G, sloping curbs are low with flat sloping faces. The curbs 
shown in Exhibits 4-6B, 4-6C, and 4-6D are considered to be mountable under emergency 
conditions although such curbs will scrape the undersides of some vehicles. For ease in crossing, 
sloping curbs should be well rounded as in Exhibits 4-6B through 4-6G. 

Extruded curbs of either cement or bituminous concrete are used in many states. Extruded 
curbs usually have sloping faces because they provide better initial stability, are easier to 
construct, and are more economical than steep faces. Typical extruded curb designs are shown in 
Exhibits 4-6C, 4-6E, and 4-6G. 

When the slope of the curb face is steeper than IV; IH, vehicles can mount the curb more 
readily when the height of the curb is limited to at most 100 mm [4 in] and preferably less. 
However, when the face slope is between IV: IH and 1V:2H, the height should be limited to 
about 150 mm [6 in]. Some highway agencies construct a vertical section on the lower face of the 
curb (Exhibits 4-6C, 4-6D, and 4-6F) as an allowance for future resurfacing. This vertical portion 
should not exceed approximately 50 mm [2 in], and where the total curb height exceeds 150 mm 
[6 in], it may be considered a vertical curb rather than a sloping curb. 

Sloping curbs can be used at median edges, to outline channelizing islands in intersection 
areas, or at the outer edge of the shoulder. For example, any of the sloping configurations in 
Exhibit 4-6 might be used for a median curb. When curbs are used to outline channelizing islands, 
an offset should be provided. Offsets to curbed islands are discussed in Chapter 9. 



324 



Cross Section Elements 



150 mm {%\tS] 



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SLOPING CURBS 



Extsibit 4-6o Typical Highway Curbs 



Shoulder curbs are placed at the outer edge of the shoulder to control drainage, improve 
delineation, control access, and reduce erosion. These curbs, combined with a gutter section, may 
be part of the longitudinal drainage system. If the surfaced shoulders are not wide enough for a 
vehicle to park, the shoulder curb should appear to be easily mountable to encourage motorists to 
park clear of the traveled way. Where it is expected that bicycHsts will use the roadway, sufficient 
width from the face of the curb should be provided so bicyclists can avoid conflict with motorists 
while not having to travel too close to the curb. For further infonnation, see the AASHTO Guide 
for the Development of Bicycle Facilities (7). 

Gutter sections may be provided on the traveled-way side of a vertical or sloping curb to 
form the principal drainage system for the roadway. Inlets are provided in the gutter or curb, or 
both. Gutters are generally 0.3 to 1.8 m [1 to 6 ft] wide, with a cross slope of 5 to 8 percent to 
increase the hydraulic capacity of the gutter section. In general, the 5 to 8 percent slope should be 
confined to the 0.6 to 0.9 m [2 to 3 ft] adjacent to the curb. Shallow gutters without a curb have 
small flow capacity and thus limited value for drainage. Generally, it is not practical to design 
gutter sections to contain all of the runoff; some overflow onto the surface can be expected. The 



325 



AASHTO — Geometric Design of Highways and Streets 



Spread of water on the traveled way is kept within tolerable limits by the proper size and spacing 
of inlets. Grate inlets and depressions for curb-opening inlets should not be placed in the lane 
because of their adverse effect on drivers who veer away from them. Bicycle-safe grates should 
be used everywhere bicyclists are peraiitted. Warping of the gutter for curb-opening inlets should 
be limited to the portion within 0.6 to 0.9 m [2 to 3 ft] of the curb to minimize adverse driving 
effects. 

The width of a vertical or sloping curb is considered a cross-section element entirely outside 
the traveled way. Also, a gutter of contrasting color and texture should not be considered part of 
the traveled way. When a gutter has the same surface color and texture as the traveled way, and is 
not much steeper in cross slope than the adjoining traveled way, it may be considered as part of 
the traveled way. This arrangement is used frequently in urban areas where restricted right-of- 
way width does not allow for the provision of a gutter. However, with any form of curb there is 
some effect on the lateral position of drivers; drivers tend to move away from a curb, which 
reduces effective through-lane width. A gutter with an evident longitudinal joint and somewhat 
steeper cross slope than the adjacent lane is a greater deterrent to driving near the gutter than the 
situation in which the traveled way and gutter are integral. 



Curb Placement 

Vertical or sloping curbs located at the edge of the traveled way may have some effect on 
lateral placement of moving vehicles, depending on the curb configuration and appearance. Curbs 
with low, sloping faces may encourage drivers to operate relatively close to them. Curbs with less 
sloping faces may encourage drivers to shy away from them and, therefore, should incorporate 
some additional roadway width. Sloping curbs placed at the edge of the traveled way, although 
considered mountable in emergencies, can be mounted satisfactorily only at reduced speeds. For 
low-speed urban street conditions, curbs may be placed at the edge of the traveled way, although 
it is preferable that the curbs be offset 0.3 to 0.6 m [1 to 2 ft]. 

Data on the lateral placement of vehicles with respect to high vertical curbs show that drivers 
will shy away from curbs that are high enough to damage the underbody and fenders of vehicles 
(4). The exact relationship is not known precisely, but it has been established that the lateral 
placement varies with the curb height and steepness and the location of other obstructions outside 
the curb. The lateral placement with respect to the curb is somewhat greater where the curb is first 
introduced than where the curb is continuous for some distance. The shying away at the 
beginning of the curb will be lessened if the curb is introduced with the end flared away from the 
pavement edge. 

Vertical curbs should not be used along freeways or other high-speed arterials, but if a curb 
is needed, it should be of the sloping type and should not be located closer to the traveled way 
than the outer edge of the shoulder. In addition, sloping-end treatments should be provided. 
Vertical curbs introduced intermittently along streets should be offset 0.6 m [2 ft] from the edge 
of the traveled way. Where a continuous curb is used along a median or channelizing island 
through an intersection or interchange, curbs should be offset at least 0.3 m [1 ft], and preferably 
0.6 m [2 ft], from the traveled way. 

326 



Cross Section Elements 



When using curbs in conjunction with traffic barriers, such as on bridges, consideration 
should be given to the type and height of barrier. Curbs placed in front of traffic barriers can 
result in unpredictable impact trajectories. If a curb is used in conjunction with a traffic barrier, 
the height of a vertical curb should be limited to 100 iTim [4 in] or it should be of the sloping type, 
ideally, located flush with or behind the face of the barrier. Curbs should not be used with 
concrete median barriers. Improperly placed curbs may cause errant vehicles to vault the concrete 
median barrier or to strike it, causing the vehicle to overturn. For a more detailed discussion on 
curb usage and location in relation to railings, refer to the AASHTO Roadside Design Guide (10). 



DRAINAGE CHANNELS AND SIDESLOPES 
General Considerations 

Modern highway drainage design should incorporate safety, good appearance, control of 
pollutants, and economical maintenance. This may be accomplished with flat sideslopes, broad 
drainage channels, and liberal warping and rounding. 

An important part of highway design is consistency, which prevents discontinuities in the 
highway environment and considers the interrelationship of all highway elements. The 
interrelationship between the drainage channel and sideslopes is important for safety because 
good roadside design can reduce the potential severity of crashes that occur when a vehicle leaves 
the roadway. 



Drainage Channels 

Drainage channels perform the vital function of collecting and conveying surface water from 
the highway right-of-way. Drainage channels, therefore, should have adequate capacity for the 
design runoff, provide for unusual storm water with minimum damage to the highway, and be 
located and shaped to provide a safe transition from the roadway to the backslope. Channels 
should be protected from erosion with the least expensive protective lining that will withstand the 
expected flow velocities. Channels should be kept clean and free of material that would lower the 
channel's capacity. Channel deterioration can reduce the capacity of the channel, which may 
result in overflow, often with erosion or deposition in the area adjacent to the channel. 

Where the construction of a highway would have an adverse effect on drainage conditions 
downstream, drainage channels can be an effective means of flood storage within the highway 
right-of-way. Drainage channels include (1) roadside channels in cut sections to remove water 
from the highway cross section, (2) toe-of-slope channels to convey the water from any cut 
section and from adjacent slopes to the natural watercourse, (3) intercepting channels placed back 
of the top of cut slopes to intercept surface water, and (4) flumes to carry collected water down 
steep cut or fill slopes. 

The primary purpose for construction of roadside channels is to control surface drainage. 
The most economical method of constructing a roadside channel usually entails the formation of 

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A ASHTO— Geometric Design of Highways and Streets 



Open-channel ditches by cutting into the natural roadside terrain to produce a drainage channel. 
From a standpoint of hydraulic efficiency, the most desirable channel contains steep sides. 
However, limitations on slope stability usually require somewhat flatter slopes. Construction and 
maintenance factors also impose restrictions on the degree of slope steepness that is practical 
alongside a highway. The offsetting factor of right-of-way costs should also be considered when 
selecting combinations of slopes to be used. 

The effect of slope combinations and safety during traversal by an errant vehicle is also an 
important consideration in designing the roadside. In general, the severity of traversal of roadside 
channels less than about 1.2 to 2.4 m [4 to 8 ft] wide is essentially the same for comparable slope 
combinations regardless of channel shape. Slope combinations forming these narrow channels 
can be selected to produce cross sections that can be safely traversed by an unrestrained vehicle 
occupant. 

The use of foreslopes steeper than 1V:4H severely limits the range of backslopes. Flatter 
foreslopes permit greater flexibility in the selection of backslopes to permit safe traversal. The 
flatter foreslope also provides greater recovery distance for an errant vehicle. For additional 
information, refer to the AASHTO Roadside Design Guide (10). 

The depth of channel should be sufficient to remove surface water without saturation of the 
subgrade. The depth of water that can be tolerated, particularly on flat channel slopes, depends 
upon the soil characteristics. In regions with severe winter climates, channel sideslopes of 1V:5H 
or IV: 6H are preferable to reduce snow drifts. 

A broad, flat, rounded drainage channel also provides a sense of openness that reduces driver 
tension. With a channel sideslope of 1V:4H or flatter and a 3.0 m [10 ft] shoulder, the entire 
roadside channel is visible to the driver. This lessens the driver's feeling of restriction and adds 
measurably to the driver's willingness to use the shoulder in an emergency. 

The minimum desirable grade for channels should be based upon the drainage velocities 
needed to avoid sedimentation. The maximum desirable grade for unpaved channels should be 
based upon a tolerable velocity for vegetation and shear on soil types. Refer to the AASHTO 
Highway Drainage Guidelines (11) for further guidance in this area. The channel grade does not 
have to follow that of the roadbed, particularly if the roadbed is flat. Although desirable, it is 
unnecessary to standardize the design of roadside drainage channels for any length of highway. 
Not only can the depth and width of the channel be varied to meet different amounts of runoff, 
slopes of channel, types of lining, and distances between discharge points, but the lateral distance 
between the channel and the edge of the traveled way can also be varied. Usually, liberal offsets 
can be obtained where cuts are slight and where cuts end and fills begin. Care should be taken, 
however, to avoid abrupt major changes in the roadway section that would result in such a 
discontinuity of the highway environment as to violate driver expectancy. Care should also be 
taken to avoid major breaks in channel grade that would cause unnecessary scour or silt 
deposition. 

Intercepting channels generally have a flat cross section, preferably formed by a dike made 
with borrow material to avoid disturbing the natural ground surface. Intercepting channels should 

328 



Cross Section Elements 



have ample capacity and should follow the contour as much as practical, except when located on 
top of a slope that is subject to sliding. In slide areas, storm water should be intercepted and 
removed as rapidly as practical. Sections of channels that cross highly permeable soil might need 
lining with impermeable material. 

Median drainage channels are generally shallow depressed areas, or swales, located at or 
near the center of the median, and formed by the flat sideslopes of the divided roadways. The 
swale is sloped longitudinally for drainage and water is intercepted at intervals by inlets or 
transverse channels and discharged from the roadway in storm drains or culverts. Flat, traversable 
drainage dikes are sometimes used to increase the efficiency of the inlets. Refer to the section on 
medians in this chapter for further discussion. Safety grates on median drains and cross drains, 
while enhancing safety for errant vehicles, can reduce the hydraulic efficiency of the drainage 
structures if not properly designed. The reduced inlet capacity is compounded by the 
accumulation of debris on the grates, occasionally resulting in roadway flooding. If the use of 
grates significantly reduces the hydraulic capacity or causes clogging problems to occur, other 
methods of drainage, or shielding of the structure, should be considered. 

Flumes are used to carry the water collected by intercepting channels down cut slopes and to 
discharge the water collected by shoulder curbs. Flumes can either be open channels or pipes. 
High velocities preclude sharp turns in open flumes and generally need some means of dissipating 
the energy of flow at the outlet of the flume. Closed flumes or pipes are preferred to avoid failure 
due to settlement and erosion. Generally in highly erodible soil, watertight joints should be 
provided to prevent failure of the facility. Caution should be exercised to avoid splash, which 
causes erosion. 

Channel erosion may be prevented with the use of linings that withstand the velocity of 
storm runoff The type of linings used in roadside channels depends upon the velocity of flow, 
type of soil, and grade and geometry of the channel. Grass is usually the most economical channel 
lining except on steep slopes where the velocity of flow exceeds the permissible velocities for 
grass protection. Other materials that can be used for channel lining where grass will not provide 
adequate protection include concrete, asphalt, stone, and nylon. Smooth linings generate higher 
velocities than rough linings such as stone and grass. Provision should be made to dissipate the 
energy of the high-velocity flow before it is released to avoid scour at the outlet and damage to 
the channel lining. If erosive velocities are developed, a special channel design or energy 
dissipater may be needed. 

Refer to the AASHTO Highway Drainage Guidelines (11) and drainage design manuals, as 
well as handbooks and publications from the Soil Conservation Service, U. S. Army Corps of 
Engineers, and Bureau of Reclamation, for details on design and protective treatments, including 
filter requirements. In addition, FHWA publications, such as Design of Stable Channels With 
Flexible Linings (12), provide excellent references. For further information on drainage design, 
see Chapter 3. 



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AASHTO — Geometric Design of Highways and Streets 



Sideslopes 

Sideslopes should be designed to ensure roadway stability and to provide a reasonable 
opportunity for recovery for an out-of-control vehicle. 

Three regions of the roadside are important to safety: the top of the slope (hinge point), the 
foreslope, and the toe of the slope (intersection of the foreslope with level ground or with a 
backslope, forming a ditch). Exhibit 4-7 illustrates these three regions. 

The hinge point contributes to loss of steering control because vehicles tend to become 
airborne in crossing this point. The foreslope region is important in the design of high slopes 
where a driver could attempt a recovery maneuver or reduce speed before impacting the ditch 
area. The toe of the slope is often within the roadside clear zone and therefore, the probability that 
an out-of-control vehicle will reach the ditch is high. In this case, a safe transition between fore- 
and backslopes should be provided. 

-d) HINGE POINT 

' ® FORESLOPE ^ ® BACKSLOPE 

'® DITCH 80TT0H 





•® TOE OF SLOPE 

Exhibit 4"7. Designation of Roadside Regions 



Research on these three regions of the roadside has found that rounding at the hinge point, 
though not essential to reduce vehicle rollovers, can increase the general safety of the roadside 
(13). Rounded slopes reduce the chances of an errant vehicle becoming airborne, thereby 
reducing the likelihood of encroachment and affording the driver more control over the vehicle. 
Foreslopes steeper than 1V:4H are not desirable because their use severely limits the choice of 
backslopes. Slopes 1V:3H or steeper are recommended only where site conditions do not permit 
use of flatter slopes. When slopes steeper than IV: 3H are used, consideration should be given to 
the use of a roadside barrier. 

Another important safety factor for intersecting roadways is the angle of break between a 
sideslope and a transverse slope. Field observations indicate that more consideration should be 
given in roadway design to carrying the desirable flat sideslopes through intersections, driveway 
approaches, median openings, and cut sections. Providing a flatter slope between the shoulder 
edge and the ditch bottom, locating the ditch a little farther from the roadway, or even enclosing 
short sections of drainage facilities will enhance the safety of the roadside, often at a small 
increase in cost. 



330 



Cross Section Elements 



Earth cut and fill slopes should be flattened and liberally rounded as fitting with the 
topography and consistent with the overall type of highway. Effective erosion control, low-cost 
maintenance, and adequate drainage of the subgrade are largely dependent upon proper shaping 
of the sideslopes. Slope and soil data are used in combination to approximate the stability of the 
slopes and the erosion potential. Overall economy depends not only on the initial construction 
cost but also on the cost of maintenance, which is dependent on slope stability. Furthermore, flat 
or rounded natural slopes with good overall appearance are appropriate for any roadside located 
near developed and populated areas. 

Normally, backslopes should be 1V:3H or flatter, to accommodate maintenance equipment, 
hi developed areas, sufficient space may not be available to permit the use of desirable slopes. 
Backslopes steeper than 1 V:3H should be evaluated with regard to soil stability and traffic safety. 
Retaining walls should be considered where space restrictions would otherwise result in slopes 
steeper than 1V:2H. On the other hand, soil characteristics may necessitate the use of slopes 
flatter than 1V:2H or even 1 V:3H. If adequate width is not available in such cases, retaining walls 
may be needed. The type of retaining structure should be compatible with the area traversed and 
the grade separation structures. To minimize the feeling of constriction, walls should be set back 
as far as practical from the traveled way. Where retaining walls are used in combination with 
earth slopes, the walls may be located either at the roadway level adjacent to the shoulder or on 
the outer portion of the separation width above the depressed roadway. 

On freeways and other arterials with relatively wide roadsides, sideslopes should be 
designed to provide a reasonable opportunity for recovery of an out-of-control vehicle. Where the 
roadside at the point of departure is reasonably flat, smooth, and clear of fixed objects, many 
potential crashes can be averted. A rate of slope of 1V:6H or flatter on embankments can be 
negotiated by a vehicle with a good chance of recovery and should, therefore, be provided where 
practical. For moderate heights with good roundings, steeper slopes up to about 1V:3H can also 
be traversable (though not recoverable). On intermediate-height fills, the cost of a continuous flat 
slope may be prohibitive, but it may be practical to provide a recovery area that is reasonably flat 
and rounded adjacent to the roadway. The recovery area should extend well beyond the edge of 
the shoulder as specific conditions may permit. 

Consistent with traffic demand, roads and streets with wide borders should also be designed 
with a similar clear roadside. However, because of generally lower speeds and narrower side 
clearances along streets, the clear roadside area concept, at best, can only be partially used. This 
is also true for widening and other reconstruction within limited right-of-way. 

Desirably, slope combinations would be selected so that unrestrained occupants could be 
expected to sustain no injury, or only minor injuries, and the vehicle would not incur major 
damage during traversal. However, site conditions such as restricted right-of-way or the cost 
effectiveness of such design may dictate the use of slope combinations steeper than desirable. If 
constraints make it impractical to provide the appropriate roadside recovery distance, the need for 
a roadside barrier should be considered. Where the height and slope of roadway embankments are 
such that the severity of potential crashes will be reduced by the placement of a roadside barrier, 
the cross section should be designed to allow adequate slope rounding and to support the barrier. 



331 



AASHTO — Geometric Design of Highways and Streets 



Flat and well-rounded sideslopes simplify the establishment of turf and its subsequent 
maintenance. Grasses usually can be readily established on sideslopes as steep as 1V:2H in 
favorable climates and 1V:3H in semiarid climates. With slopes of 2V:3H and steeper, it is 
difficult to establish turf, even in areas of abundant rainfall. Because of the greater velocity of 
runoff, sufficient water for the maintenance of grass does not seep into the soil. Deep-rooted 
plants that do not depend upon surface water alone may be appropriate where slopes are 
excessively steep. Slopes of the order of IV: 3H and flatter can be mechanically mowed. Although 
steeper slopes reduce the mowing area considerably, the slow, time-consuming manual methods 
required to mow the area add substantially to maintenance costs. 

With some types of soils, it is essential for stability that slopes be reasonably flat. Soils that 
are predominantly clay or gumbo are particularly susceptible to erosion, and slopes of IV: 3H or 
flatter should be used. The intersections of slope planes in the highway cross section should 
complement the earth forms of the terrain being traversed. Some earth forms are well-rounded 
and others are steeply sloped. The designer should strive to create a natural look that is 
aesthetically pleasing. Since rounded landforms are the natural result of erosion, such rounded 
forms are stable; therefore, use of well-rounded forms in the design of the highway cross section 
is likely to result in stability. 

To attain a natural appearance along the roadside, flat, well-rounded sideslopes should be 
provided. A uniform slope through a cut or fill section often results in a formal or stilted 
appearance. This appearance can be softened and made more natural by flattening the slopes on 
the ends where the cut or fill is minimal and by gradually steepening it toward the controlling 
maximum slope of the cut or fill. This design may be readily accomplished by liberal rounding of 
the hinge point in the transition area. On short cut or fill sections the result may be one of 
continuous longitudinal rounding whereas, on sections of substantial length the effect will be one 
of funneling. The transitioning of sideslopes is especially effective at the ends of cuts when 
combined with an increased lateral offset of the drainage channel and a widened shoulder. 

The combination of flat slopes and rounding is frequently referred to as a streamlined cross 
section. With this shape, the cross winds sweep along the surface without forming eddies that 
contribute to the wind erosion and drifting of snow. The streamlined cross section usually results 
in a minimum expenditure for snow removal because the winds blow the snow off the traveled 
way instead of drifting it, as happens in cross sections with steep slopes and no rounding. When 
combined with the design of an elevated roadway on earth embankment to ensure drainage of the 
subgrade, the streamlined cross section results in a roadway that needs minimal maintenance and 
operating costs and operates safely. 

In some cases, an irregular slope stake line results from the strict adherence to specified cut 
or fill slopes. It may be more aesthetically pleasing to vary the slope to yield a neat stake line. 

Design slopes for rock vary widely, depending upon the materials. A commonly used slope 
for rock cuts is 2V:1H. With modem construction methods, such as pre-splitting, slopes ranging 
as steep as 6V:1H may be used in good-quality rock. Deep cuts in rock often require the 
construction of benches in the slopes. 



332 



Cross Section Elements 



Slope Stability as well as appearance may be enhanced in poor-quality rock by the 
establishment of vegetative cover. In some parts of the country, serrated cut slopes aid in the 
establishment of vegetative cover on decomposed rock or shale slopes. Serration may be 
constructed in any material that can be ripped or that will hold a vertical face long enough to 
establish vegetation (14). 

Desirably, the toe of the rock-cut slope should be located beyond the minimum lateral 
distance from the edge of the traveled way needed by the driver of an errant vehicle to either 
regain control and begin a return to the roadway or to slow the vehicle. Wide shelves at the 
bottom of rock cuts have advantages in that a safe landing area is provided for falling boulders 
and space is available for snow storage in colder climates. This width can also be shaped to 
provide a clear roadside recovery area. 

Rock outcroppings are frequently left in place during construction of new highways for 
economic or aesthetic purposes. These should be eliminated within the clear roadside recovery 
area where removal is practical. Alternatively, if they cannot be removed, they should be shielded 
by the installation of a roadside barrier. 

For additional guidance on sideslope design, refer to the AASHTO Roadside Design 
Guide (10). 



ILLUSTRATtVE OUTER CROSS SECTIONS 

Exhibits 4-1 and 4-2 illustrate typical combinations of outer cross-section elements — 
shoulders, side-drainage channels, sidewalks, curbs, and sideslopes — for normal crowned and 
superelevated sections, respectively. Only a few of the desirable arrangements are illustrated, but 
other practical arrangements are discussed. 



Normal Crown Sections 

Exhibit 4-lA shows the most widely used cross section in modern highway practice. The 
combination of elements is simple and forms a streamlined cross section. Usable shoulder widths 
are included on both the fill and cut sections. The controlling shoulder slopes range from 
2 percent, for a paved or impervious surface, to 8 percent, the maximum slope applicable to a turf 
surface. 

In Exhibit 4-1 A the drainage channel at the right is formed by the foreslope on the roadway 
side and the cut slope, or backslope, on the outer side. The foreslope and backslope combination 
should be designed such that it can be safely traversed by an errant vehicle. The channel should 
be wide enough to provide sufficient drainage capacity and deep enough to ensure roadbed 
stability. A depth of 0.3 to 1.2 m 11 to 4 ft] below the shoulder break is recommended. 

In areas where an errant vehicle may tend to encroach the roadside, it is desirable to provide 
rounding at the intersection of slope planes. Rounding of all slope intersections also improves 

333 



AASHTO — Geometric Design of Highways and Streets 



appearance and simplifies maintenance. In general, 1.2 to 1.8 m [4 to 6 ft] of rounding is the 
minimum desirable at the edge of the shoulder. The rounding needed at the top of cut slopes is 
dependent upon a number of factors, including the type of soil, slope ratio and height, and the 
natural ground slopes. The rounding may vary from 1.2 m [4 ft] to 4.5 m [15 ft]. Toe-of-slope 
rounding minimizes slope change and offers an increase in fill stability. Toe-of-slope rounding 
also varies with slopes and fill heights, and has the same general dimensions as on cut slopes. 

Exhibit 4-lB illustrates a type of curb treatment that can be used for drainage control or to 
separate roadways and sidewalks. The left side of the exhibit shows a curb or dike that is used for 
fill slope protection. The shoulder slope of this section should be designed in conjunction with the 
drainage system to prevent ponding upon the roadway. Frequent outlets are needed for drainage. 
To the extent possible, sidewalks should be separated from the roadway. In areas fully developed 
with retail stores and offices, it may not be practical to offset the sidewalk from the roadway 
because of the right-of-way considerations. In such cases, curbs are used to separate the sidewalk 
from the edge of the roadway. This section is shown on the right side of the exhibit. 

Exhibit 4-lC shows a steep fill section with guardrail at the edge of the shoulder on the left 
side of the roadway. Where a sidewalk is needed, it should be located behind the guardrail. For 
shallow fill sections, roadside safety may be enhanced by enclosing sections of drainage facilities, 
as shown on the right side of the roadway. 



Superelevated Sections 

The low sides of the three superelevated cross sections of Exhibit 4-2 are similar to those of 
Exhibit 4-1 except for the shoulder slope in those cases where the superelevation rate is greater 
than the normal shoulder slope. It is desirable from an operational standpoint that the shoulder 
slope on the low side be the same as the traveled way superelevation slope. 

In Exhibit 4-2A the direction of shoulder slope on the high side of the cross section is the 
same as that for normal crowned traveled ways except that its rate of slope should be limited. To 
avoid an undesirable rollover effect, the algebraic difference in cross slopes at the edge of the 
traveled way should not exceed 8 percent. Accordingly, use of this cross section should be 
reserved for low rates of superelevation and shoulder slope. The shoulder slope on the alternate 
section of Exhibit 4-2A is a projection of the superelevated traveled way. 

In Exhibit 4-2B the level shoulder on the high edge of this cross section represents a 
compromise that prevents the shoulder from draining to the traveled way while complying with 
the 8 percent rollover control. The use of this cross section should be reserved for stable soils 
where the percoladon, caused by the water falling directly upon the shoulder, is not very great. 
Where snowfall is prevalent, this cross section would tend to allow snow melt from a windrow on 
the shoulder to flow across the traveled way, creating a potential icing situation when refreezing 
occurs. 

Exhibit 4-2C shows the high-side shoulder rolled over in a well-rounded transverse vertical 
curve so that the water falhng upon the shoulder is divided between the traveled way and the side 

334 



Cross Section Elements 



channel or fill slope. On this rounded shoulder, any vehicle would stand nearly level as needed to 
facilitate tire changes and other repairs. The vertical curve should not be less than 1.2 m [4 ft] 
long, and at least the inner 0.6 m [2 ft] of the shoulder should be held at the superelevated slope. 
The shoulder slope on the alternate section of Exhibit 4-2C is a planar section with multiple 
breaks. 

Superelevation is advantageous for traffic operations on less developed arterial s, as well as 
for rural highways and urban freeways; however, in built-up areas, the combination of wide 
pavements, proximity of adjacent development, control of cross slope and profile for drainage, 
frequency of cross streets, and other urban features combine to make superelevation impractical 
or undesirable. Usually, superelevation is not provided on local streets in residential, commercial, 
or industrial areas. For further information on superelevation, refer to Chapter 3. 



TRAFFIC BARRIERS 
General Considerations 

Traffic barriers are used to prevent vehicles that leave the traveled way from hitting an 
object that has greater crash severity potential than the barrier itself Because barriers are a source 
of crash potential themselves, their use should be carefully considered. For more detailed 
information regarding traffic barriers, refer to the AASHTO Roadside Design Guide (10). 

Research continues to develop improved and more cost-effective barriers. The criteria 
discussed herein will undoubtedly be refined and amended in the future. Therefore, the designer 
should remain current on new barrier concepts and criteria. 

Traffic barriers include both longitudinal barriers and crash cushions. The primary function 
of longitudinal barriers is to redirect errant vehicles. The primary function of crash cushions is to 
decelerate errant vehicles to a stop. 

Longitudinal barriers are located along the roadside and in medians. Bridge parapets or rails 
are covered in AASHTO design criteria and specifications for highway bridges. Longitudinal 
barriers are generally denoted as one of three types: flexible, semirigid, or rigid. The major 
difference between these types is the amount of barrier deflection that takes place when the 
barrier is struck. 

Flexible barrier systems undergo considerable dynamic deflection upon impact and generally 
impose lower impact forces on the vehicle than semirigid and rigid systems. The resistance of this 
system is derived from tensile force in the longitudinal member. Within the impact zone, the 
cable or beams tear away from the support post upon impact; thus, the post offers negligible 
resistance. However, the posts outside the impact zone provide sufficient resistance to keep the 
deflection of the longitudinal member within an acceptable limit. This system is designed 
primarily to contain rather than redirect the vehicle and needs more lateral clearance from fixed 
objects due to the deflection during impact. 



335 



AASHTO — Geometric Design of Highways and Streets 



In the semirigid system, resistance is achieved through the combined flexure and tensile 
strength of the rail. The posts near the point of impact are designed to break or tear away, thereby 
distributing the impact force by beam action to adjacent posts. However, posts outside the impact 
zone provide sufficient resistance to control the deflection of the longitudinal member to an 
acceptable limit and redirect the errant vehicle along the path of traffic flow. 

A rigid system does not deflect substantially upon impact. During collisions, energy is 
dissipated by the raising and lowering of the vehicle and by deformation of the vehicle sheet 
metal. As the angle of impact increases, barrier deceleration forces increase because of the 
absence of barrier deflection. Therefore, installation of a rigid system is most appropriate where 
shallow impact angles are expected, such as along narrow medians or shoulders. The rigid system 
has proved to be very effective as a protective shield where deflection cannot be tolerated, such as 
at a work zone. Because this system suffers little or no damage on impact, hence needing little 
maintenance, it should be considered where heavy traffic volumes hamper replacement of 
damaged rail. 

Important factors to consider in the selection of a longitudinal system include barrier 
performance, lateral deflection characteristics, and the space available to accommodate barrier 
deflection. Consideration should also be given to the adaptability of the system to operational 
transitions and end treatments and to the initial and future maintenance cost. 

Six options are available for the treatment of roadside obstacles: (1) remove or redesign the 
obstacle so it can be safely traversed, (2) relocate the obstacle to a point where it is less likely to 
be struck, (3) reduce impact severity by using an appropriate breakaway device, (4) redirect a 
vehicle by shielding the obstacle with a longitudinal traffic barrier and/or crash cushion, (5) 
delineate the obstacle if the above alternatives are not appropriate, or (6) take no action. 

Roadway cross section significantly affects traffic barrier performance. Curbs, dikes, sloped 
shoulders, and stepped medians can cause errant vehicles to vault or submarine a barrier or to 
strike a barrier so that the vehicle overturns. Optimum barrier system performance is provided by 
a relatively level surface in front of the barrier and, for semirigid and flexible barriers, beneath 
and behind the barrier. Where curbs and dikes are used to control drainage, they should be located 
flush with the face of the barrier or slightly behind it. 

In new construction, all curbs and dikes that are not an integral part of the barrier system 
should be avoided; drainage should be controlled by gentle swales or other means that will not 
adversely affect barrier performance. Where a barrier is to be installed in the vicinity of an 
existing curb and the cost of removing the curb cannot be justified, the designer should select a 
barrier and locate it so that the adverse effect of the curb on barrier performance is minimized. 



336 



Cross Section Elements 



Longitudinal Barriers 

Roadside Barriers 

A roadside barrier is a longitudinal system used to shield motorists from obstacles or slopes 
located along either side of a roadway. It may occasionally be used to protect pedestrians, 
bystanders, and cyclists from vehicular traffic. Elements which may warrant shielding by a 
roadside barrier include embankment obstacles, roadside obstacles, and sensitive areas such as 
playgrounds. 

Recent studies indicate that rounding at the shoulder and at the toe of an embankment slope 
can reduce its crash severity potential. Rounded slopes reduce the chances that an errant vehicle 
will become airborne, thereby reducing the potential consequences of an encroachment and 
affording the driver more vehicle control. 

The height and slope of an embankment are the key factors in determining barrier need 
through a fill section. The designer should refer to current warrants and criteria for determination 
of barrier needs (10). 

A clear, unobstructed, flat roadside is desirable. When these conditions do not exist, criteria 
to determine the need for a barrier should be consulted. Roadside obstacles include non- 
traversable areas and fixed objects. If it is not practical to remove, modify, or relocate an obstacle, 
then a barrier may be needed. The purpose of a barrier is to enhance safety. Therefore, a barrier 
should be installed only if it is clear that the barrier will have lower crash severity potential than 
the roadside obstacle. 

Short lengths of roadside barriers are discouraged. Where a barrier is needed in two or more 
closely spaced locations, continuous barrier should be provided. 

Barriers should be located beyond the edge of the shoulder to ensure that the full shoulder 
width may be used. The fill supporting the barrier should be sufficiently wide to provide lateral 
support. At bridge locations, roadside barriers should be aligned with the bridge rail and properly 
secured to the bridge to minimize the possibility of a vehicle striking the barrier and snagging or 
colliding with a bridge rail or curb. Proper treatment of the exposed end of the barrier is also 
important. An untreated or square approach end of a barrier presents a formidable roadside 
obstacle. To provide safe barriers, ends may be buried, covered with a mound of earth, flared 
back, or protected with a crash cushion or an approved crash tested terminal. Buried barrier ends 
should be designed to minimize ramping of impacting vehicles. The AASHTO Roadside Design 
Guide (10) provides more information on crashworthy end treatments. 

The need for a barrier in rock cuts and near large boulders is a matter of judgment by the 
highway designer and depends on the potential severity of a crash and the lateral clearance 
available. 

For additional material on roadside barriers, refer to the AASHTO Roadside Design Guide 

(10). 

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AASHTO — Geometric Design of Highways and Streets 



Median Barriers 

A median barrier is a longitudinal system used to minimize the possibility of an errant 
vehicle crossing into the path of traffic traveling in opposite directions. When traffic volumes are 
low, the probability of a vehicle crossing a median and colliding with a vehicle in the opposing 
direction is relatively low. Likewise, for relatively wide medians the probability of a vehicle 
crossing the median and colliding with a vehicle in the opposing roadway is also relatively low. 
In these instances, median barriers are generally recommended only when there has been a 
history of cross-median collisions or, for new roadways, where an incidence of high crash rates of 
this type would be expected. Although cross-median collisions may be reduced by median 
barriers, total crash frequency will generally increase because the space available for retum-to- 
the-road maneuvers is decreased. 

Special consideration should be given to barrier needs for medians separating traveled ways 
at different elevations. The ability of an errant driver leaving the higher elevated roadway to 
return to the road or to stop diminishes as the difference in elevations increases. Thus, the 
potential for cross-median head-on collisions increases. 

An important safety consideration in the design of median barriers is shielding motorists 
from the exposed end of the barrier. As discussed previously, exposed ends may be buried, 
covered with a mound of earth, flared back, or protected with an end terminal end or a crash 
cushion. For more information on crashworthy end treatments, refer to the AASHTO Roadside 
Design Guide (10). 

For all divided highways, regardless of median width and traffic volume, the median 
roadside should also be examined for other factors, such as obstacles and lateral drop-off, as 
discussed earlier. 

Careful consideration should be given to the installation of median barriers on multilane 
expressways or other highways with partial control of access. Even medians that are narrow 
permit inadvertent encroachments with a chance for motorist recovery and can also include 
geometric features to accommodate crossing or left-tum traffic. With the addition of a barrier, 
barrier ends at median openings present formidable obstacles. Crash cushions, although needing 
maintenance and imposing a high initial cost, may be needed to shield an errant motorist from 
barrier ends. Consequently, an evaluation of the number of median openings, crash history, 
alignment, sight distance, design speed, traffic volume, and median width should be conducted 
prior to installation of median bairiers on non-freeway facilities. 

Barriers should also be considered on outer separations of 15 m [50 ft] or less where the 
frontage roads carry two-way traffic. 

Common types of median barrier include double-faced steel W-beam (blocked-out) installed 
on strong posts, box beam installed on weak posts, and concrete barrier. Less common types of 
median barrier include two- or three-cable banier installed on light steel posts, double-faced steel 
W-beam installed on weak posts, double-faced steel three-beam (blocked-out) installed on strong 



338 



Cross Section Elements 



posts, and a cable-chainlink fence combination. For additional data on median barrier types, refer 
to the AASHTO Roadside Design Guide (10). 

In selecting the type of median barrier, it is important to match the dynamic lateral deflection 
characteristics to the site. The maximum deflection should be less than one-half the median width 
to prevent penetration into the opposing lanes of traffic. The median barrier should be designed to 
redirect the colliding vehicle in the same direction as the traffic flow. In addition, the design 
should be aesthetically pleasing. 

On heavily traveled facilities, a concrete barrier with a sloping face has many advantages. 
For example, this type of barrier deflects a vehicle striking it at a slight impact angle. It is 
aesthetically pleasing and needs little maintenance. The latter is an important consideration on 
highways with narrow medians since maintenance operations encroach on the high-speed traveled 
way and may require closure of one of the traffic lanes during repair time. The designer should 
also bear in mind that even though a concrete barrier does not deflect, there may be significant 
intrusion into the air space above and beyond the barrier by high-center-of-gravity vehicles 
striking the barrier at high speeds or large angles. A bus or tractor-trailer may lean enough to 
strike objects mounted on top of the barrier or within a distance of up to 3.0 m [10 ft] of the 
barrier face. While piers and abutments may be able to withstand such impacts, other structures 
such as sign trusses and luminaire supports may become involved in secondary collisions. 

The appropriate types of median barriers are different for stepped median sections 
(i.e., where the median is between roadways of different elevations). Cable, W-beam on weak 
posts, and box-beam systems are generally limited to relatively flat medians and may not be 
appropriate for some stepped median sections. The AASHTO Roadside Design Guide (10) 
provides further guidance in this area. 

It is important that, during the selection and design of a median barrier, consideration is 
given to the potential effect of the barrier on sight distance on horizontal curves. 

Due to ongoing research and development, the design of median barriers and terminals is 
continually improving. Reference should be made to the latest developments in median barrier 
and terminal design. 

Precast concrete median barrier can be used for temporary protection of work areas and for 
guiding traffic during construction. It can also be incorporated permanently as part of the 
completed facility. 



Bridge Railings 

Bridge railings prevent vehicles, pedestrians, or cyclists from falling off the structure. 
AASHTO' s Standard Specifications for Highway Bridges (15) specifies geometric, design load, 
and maximum allowable material stress requirements for the design of traffic railings for 
pedestrians, bicycles, and combination types. Bridge railings are longitudinal traffic barriers that 



339 



AASHTO — Geometric Design of Highways and Streets 



differ from other traffic barriers primarily in their foundations. These railings are a structural 
extension of a bridge while other traffic barriers are usually set in or on soil. 

The need for a traffic barrier rarely ends at the end of a bridge. Therefore, the bridge railing 
should be extended with a roadside barrier, which in turn should have a crash-worthy terminal. At 
the juncture between a bridge railing and roadside barrier, an incompatibility usually exists in the 
stiffness of the two barrier types. This stiffness should be carefully transitioned over a length to 
prevent the barrier system from pocketing or snagging an impacting vehicle. 

Where a roadside barrier is provided between the edge of the traveled way and the bridge 
railing so that a sidewalk can be included, special attention should be given to the barrier end 
treatment. End treatments that are both functional and safe are difficult to design. The end 
treatments should safely accommodate vehicles, yet not impede pedestrian usage of the walkway. 

The recommended lateral clearances between the traveled way and bridge railings usually 
exceed curb offset distances. This may create a problem in a bridge railing where a curbed cross 
section is used on a bridge approach and a flush cross section is used on the bridge. Such 
problems may result if the length of the bridge and its approaches make the bridge resemble a 
controlled-access facility where traffic will operate at speeds in excess of 80 km/h [50 mph], even 
though the approach speeds are less than 80 km/h [50 mph]. Such high speeds may render curb 
usage acceptable away from but not on the bridge. In such cases, it may be reasonable to drop the 
curb at the first intersection away from the end of the bridge. Another option is to reduce the curb 
to a low, sloping curb with a gently sloped traffic face, well in advance of the introduction of the 
traffic barrier. This would be reasonably compatible with the traffic barrier even if continued into 
the high-speed region of the bridge. 



Crash Cushions 

Crash cushions are protective systems that prevent errant vehicles from impacting roadside 
obstacles by decelerating the vehicle to a safe stop when hit head-on or redirecting it away from 
the obstacle (10). A coirmion application of a crash cushion is at the end of a bridge rail located in 
a gore area. Where site conditions permit, a crash cushion should also be considered as an 
alternative to a roadside barrier for shielding rigid objects such as bridge piers, overhead sign 
supports, abutments, and retaining-wall ends. Crash cushions may also be used to shield roadside 
and median barrier terminals. 

Site preparation is important in using crash cushion design. Inappropriate site conditions 
may compromise cushion effectiveness. Crash cushions should be located on a level area free 
from curbs or other physical obstacles. The design of new highway facilities should consider 
alternatives to use of crash cushions where appropriate. 



340 



Cross Section Elements 



MEDIANS 

A median is the portion of a highway separating opposing directions of the traveled way. 
Medians are highly desirable on arterials carrying four or more lanes. Median width is expressed 
as the dimension between the edges of traveled way and includes the left shoulders, if any. The 
principal functions of a median are to separate opposing traffic, provide a recovery area for out- 
of-control vehicles, provide a stopping area in case of emergencies, allow space for speed 
changes and storage of left-turning and U-tuming vehicles, minimize headlight glare, and provide 
width for future lanes. Additional benefits of a median in an urban area are that it may offer an 
open green space, may provide a refuge area for pedestrians crossing the street, and may control 
the location of intersection traffic conflicts. For maximum efficiency, a median should be highly 
visible both night and day and should contrast with the traveled way. Medians may be depressed, 
raised, or flush with the traveled way surface. 

In determining median width, consideration should be given to the potential need for median 
barrier. Where practical, median widths should be such that a median barrier is not needed. The 
general range of median widths is from 1.2 to 24 m [4 to 80 ft] or more. Economic factors often 
limit the median width that can be provided. Cost of construction and maintenance increases as 
median width increases, but the additional cost may not be appreciable compared with the total 
cost of the highway and may be justified in view of the benefits gained. 

At unsignalized intersections on rural divided highways, the median should generally be as 
wide as practical. In urban and suburban areas, however, narrower medians appear to operate 
better at unsignalized intersections; therefore, wider medians should only be used in urban and 
suburban areas where needed to accommodate turning and crossing maneuvers by larger vehicles 
(16). Medians at unsignalized intersections should be wide enough to allow selected design 
vehicles to safely make a selected maneuver. The appropriate design vehicle for determining the 
median width should be chosen based on the actual or anticipated vehicle mix of crossroad and 
U-tum traffic. A consideration in the use of wider medians on roadways other than freeways is 
the provision of adequate storage area for vehicles crossing the highway at unsignalized 
intersections and at median openings serving commercial and private driveways. Such median 
openings may need to be controlled as intersections (see Chapter 9). Wide medians may be a 
disadvantage when signalization is needed. The increased time for vehicles to cross the median 
can lead to inefficient signal operation. 

If right-of-way is restricted, a wide median may not be justified if provided at the expense of 
narrowed border areas. A reasonable border width is needed to adequately serve as a buffer 
between the private development along the road and the traveled way, particularly where zoning 
is limited or non-existent. Space should be provided on the borders for sidewalks, highway signs, 
utility lines, parking, drainage channels, structures, proper slopes, clear recovery zones, and any 
retained native growth. Narrowing the border areas may create obstacles and hindrances similar 
to those that the median is designed to avoid. 

A depressed median is generally preferred on freeways for more efficient drainage and snow 
removal. Median side slopes should preferably be 1V:6H, but slopes of 1V:4H may be adequate. 



341 



AASHTO — Geometric Design of Highways and Streets 



Drainage inlets in the median should be designed either with the top of the inlet flush with the 
ground or with culvert ends provided with traversable safety grates. 

Raised medians have application on arterial streets where it is desirable to regulate left-turn 
movements. They are also frequently used where the median is to be planted, particularly where 
the width is relatively narrow. Careful consideration should be given to the location and type of 
plantings. Plantings, particularly in nan^ow medians, may create problems for maintenance 
activities. Also, plantings such as trees in the median can also cause visual obstructions for 
turning motorists if not carefully located. Plantings and other landscaping features in median 
areas may constitute roadside obstacles and should be consistent with the AASHTO Roadside 
Design Guide (10). 

Flush medians are commonly used on urban arterials. Where used on freeways, a median 
barrier may be needed. The crowned type is frequently used because it eliminates the need for 
collecting drainage water in the median. In general, however, the slightly depressed median is 
preferred either with a cross slope of about 4 percent or with a minor steepening of the roadway 
cross slope. 

The concept of converting flush medians to two-way left-turn lanes on urban streets has 
become widely accepted. This concept offers several advantages when compared to no median. 
Among these advantages are reduced travel time; improved capacity; reduced crash frequency, 
particularly of the rear-end type; more flexibility (because the median lane can be used as a travel 
lane during closure of a through lane); and public preference both from drivers and owners of 
abutting properties (33). Median widths of 3.0 to 4.8 m [10 to 16 ft] provide the optimum design 
for two-way left-turn lanes. Refer to the MUTCD (8) for appropriate lane markings and to 
Chapter 2 for additional discussion and details. 

Two-way left-turn lanes may be inappropriate at many locations and conversion of existing 
two-way left-turn lanes to nontraversable medians should be considered. Two-way left-turn lanes 
have been widely used to provide access to closely spaced, low-volume commercial driveways 
along arterial roads. From an access management perspective, they increase rather than control 
access opportunities. Highway agencies have installed raised-curb or concrete median barriers on 
existing highways in place of flush medians to better manage highway access as traffic and safety 
concerns increase. In addition, some median openings for minor streets have been closed, 
permitting only right turns in and out of these streets. This median treatment can reduce the 
number and location of conflicts along a section of roadway. It should be recognized that diverted 
left-turn volumes may increase congestion and collisions at downstream intersections; provisions 
to accommodate U-turn traffic should also be considered at downstream locations. 

Where there is no fixed-source lighting, headlight glare across medians or outer separations 
can be a nuisance, particularly where the highway has relatively sharp curves or if the profiles of 
the opposing roadways are uneven. Under these conditions, some form of antiglare treatment 
should be considered as part of the median barrier installation, provided it does not act as a snow 
fence and does not create drifting problems. 



342 



Cross Section Elements 



When medians are about 12 m [40 ft] or wider, drivers have a sense of separation from 
opposing traffic; thus, a desirable ease and freedom of operation is obtained, the noise and air 
pressure of opposing traffic is not noticeable, and the glare of headlights at night is greatly 
reduced. With widths of 18 m [60 ft] or more, the median can be pleasingly landscaped in a park- 
like manner. Plantings used to achieve this park-like appearance need not compromise the 
roadside recovery area. 

There is demonstrated benefit in any separation, raised or flush. Wider medians are desirable 
at rural unsignalized intersections, but medians as wide as 18 m [60 ft] may not be desirable at 
urban and suburban intersections or at intersections that are signalized or may need signalization 
in the foreseeable future. For further guidance in the selection of median widths for divided 
highways with at-grade intersections, refer to NCHRP Report 375, Median Intersection 
Design (16). 



FRONTAGE ROADS 

Frontage roads serve numerous functions, depending on the type of arterial they serve and 
the character of the surrounding area. They may be used to control access to the arterial, function 
as a street facility serving adjoining properties, and maintain circulation of traffic on each side of 
the arterial. Frontage roads segregate local traffic from the higher speed through-traffic and 
intercept driveways of residences and commercial establishments along the highway. Cross 
connections provide access between the traveled way and frontage roads and are usually located 
in the vicinity of the crossroads. Thus, the through character of the highway is preserved and 
unaffected by subsequent development of the roadsides. 

Frontage roads are used on all types of highways. Each chapter pertaining to a particular 
type of highway includes a discussion on the use of frontage roads with that highway type. 
Frontage roads are used most frequently on freeways where their primary function is to distribute 
and collect traffic between local streets and freeway interchanges. In some circumstances, 
frontage roads are desirable on arterial streets both in downtown and suburban areas. Frontage 
roads not only provide more favorable access for commercial and residential development than 
the faster moving arterial street but also help to preserve the safety and capacity of the latter. In 
rural areas, development of expressways may need separated frontage roads that are somewhat 
removed from the right-of-way and serve as access connections between crossroads and adjacent 
farms or other development. 

Despite the advantages of using frontage roads on arterial streets, the use of continuous 
frontage roads on relatively high-speed arterial streets with intersections may be undesirable. 
Along cross streets, the various through and turning movements at several closely spaced 
intersections may greatly increase crash potential. Multiple intersections are also vulnerable to 
wrong-way entrances. Traffic operations are improved if the frontage roads are located a 
considerable distance from the main line at the intersecting cross roads in order to lengthen the 
spacing between successive intersections along the crossroads. In urban areas, a minimum 
spacing of about 50 m [150 ft] between the arterial and the frontage roads is desirable. For further 



343 



AASHTO — Geometric Design of Highways and Streets 



discussion on frontage roads at intersections, refer to the section in Chapter 9 on "Intersection 
Design Elements With Frontage Roads." 

In general, frontage roads are parallel to the traveled way, may be provided on one or both 
sides of the arterial, and may or may not be continuous. Where the highway crosses a grid street 
system on a diagonal course or where the street pattern is irregular, the frontage roads may be a 
variable distance from the traveled way. Arrangements and patterns of frontage roads are shown 
in Exhibits 4-8 and 4-9. Exhibit 4-8A illustrates the most common arrangement, two frontage 
roads running parallel and approximately equidistant from a freeway. In urban areas, continuous 
frontage roads that are parallel to the freeway permit the use of the frontage roads as a backup 
system in case of an accident on the freeway or other freeway disruption. Exhibit 4-8B shows a 
freeway with one frontage road. On the side without the frontage road, the local streets serve to 
collect and distribute the traffic. Exhibit 4-9 shows an irregular pattern of frontage roads. 



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Exhibit 4-8. Typical Frontage Road Arrangements 



From an operational and safety standpoint, one-way frontage roads are much preferred to 
two-way frontage roads. While one-way operation inconveniences local traffic to some degree, 
the reduction in vehicular and pedestrian conflicts at intersecting streets generally compensate for 
this inconvenience. In addition, there is some reduction in the roadway and right-of-way width 
required. Two-way frontage roads at busy intersections complicate crossing and turning 



344 



Cross Section Elements 



movements. Where offramps join a two-way frontage road, the potential for wrong-way entry is 
increased. This problem is greatest where the ramp joins the frontage road at an acute angle, thus 
giving the appearance of an onramp to the wrong-way driver. 

Two-way frontage roads may be considered for partially developed urban areas where the 
adjoining street system is so irregular or so disconnected that one-way operation would introduce 
considerable added travel distance and cause undue inconvenience. Two-way frontage roads may 
also be appropriate for suburban or rural areas where points of access to the through facility are 
infrequent, where only one frontage road is provided, or where roads or streets connecting with 
the frontage roads are widely spaced, hi urban areas that are developed or likely to be developed, 
two-way frontage roads should be considered where there is no parallel street within reasonable 
distance of the frontage roads. 

Connections between the arterial and frontage road are an important element of design. On 
arterials with slow-moving traffic and one-way frontage roads, slip ramps or simple openings in a 
narrow outer separation may work reasonably well. Slip ramps from a freeway to two-way 
frontage roads are generally unsatisfactory because they may induce wrong-way entry to the 
freeway traveled way and create an increased crash potential at the intersection of the ramp and 
frontage road. On freeways and other arterials with high operating speeds, the ramps and their 
terminals should be liberally designed to provide for speed changes and storage. Details of ramp 
design are covered in later chapters. 

Exhibits 4-10 and 4-11 each illustrate an arrangement of frontage roads with entrance and 
exit ramps that are applicable to freeways and other higher speed arterials. The one-way frontage 
roads illustrated in Exhibit 4-10 are designed to ensure good operation on both freeways and 
frontage roads. Exhibit 4-11 shows an arrangement of entrance and exit ramps at two-way 
frontage roads. This design incorporates a wide outer separation that is not always practical in 




Exhibit 4-9, Frontage Roads, Irregular Pattern 



345 



AASHTO — Geometric Design of Highways and Streets 



urban areas. The actual width would depend on the design of the ramps and their terminals. In 
most cases, the width of outer separation would be greater than 60 m [200 ft] in the area of the 
ramp terminals. The offramp is connected to the frontage road at a right angle to discourage 
wrong-way entry. Careful attention needs to be given to the placement of signs and the use of 
traffic markings to prohibit wrong-way movements. Because of the potential for wrong-way 
movements, the offramp should not intersect the frontage road opposite a two-way side street 
access. 



:2^ 




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Exhibit 4-10p One-way Frontage Roads, Entrance and Exit Ramps 



The design of a frontage road is influenced by the type of service it is intended to provide. 
Where a frontage road is continuous and passes through highly developed areas, it assumes the 
character of an important street, serving both local traffic as well as overflow from the traveled 
way. Where the frontage roads are not continuous or are only a few blocks in length, follow an 
irregular pattern, border the rear and sides of buildings, or serve only scattered development, 
traffic will be light and operation will be local in character. Refer to Chapter 6 for guidelines on 
the widths of two-lane frontage roads for rural and urban collectors. 



Frt>nta9eft«»^<^*^ 




Exhibit 4- lie Two-way Frontage Roads, Entrance and Exit Ramps 



OUTER SEPARATIONS 

The area between the traveled way of a through-traffic roadway and a frontage road or street 
is referred to as the "outer separation." Such separations function as buffers between the through- 



346 



Cross Section Elements 



traffic on the arterial and the local traffic on the frontage road and provide space for a shoulder 
for the through roadway and ramp connections to or from the through facility. 

The wider the outer separation, the less influence local traffic will have on through-traffic. 
Wide separations lend themselves to landscape treatment and enhance the appearance of both the 
highway and the adjoining property. A substantial width of outer separation is particularly 
advantageous at intersections with cross streets because it minimizes vehicle and pedestrian 
conflicts. 

Where ramp connections are provided between the through roadway and the frontage road, 
the outer separation should be substantially wider than typical. The needed width will depend 
mostly upon the design of the ramp termini. 

Where two-way frontage roads are provided, a driver on the through facility faces 
approaching traffic on the right (opposing frontage road traffic) as well as opposing arterial traffic 
on the left. Desirably, the outer separation should be sufficiently wide to minimize the effects of 
approaching traffic, particularly the potentially confusing and distracting nuisance of headlight 
glare at night. With one-way frontage roads the outer separation need not be as wide as with two- 
way frontage roads. 

The one-lane, one-way frontage road with parking illustrated in Exhibit 4-12 serves 
businesses along a major undivided arterial street in a densely developed area of a large city. The 
raised and curbed outer separation creates a buffer between through-traffic and local traffic and 
provides a refuge for pedestrians. 




Exhibit 4-12» Frontage Road in Business Area With Narrow Outer Separation 



The cross section and treatment of an outer separation depend largely upon its width and the 
type of arterial and frontage road. Preferably, the strip should drain away from the through 
roadway either to a curb and gutter at the frontage road or to a swale within the strip. Typical 
cross sections of outer separations for various types of arterials are illustrated in Exhibit 4-13. 

347 



AASHTO — Geometric Design of Highways and Streets 



The cross section in Exhibit 4-13A is applicable to low-speed arterial streets in densely 
developed areas. Exhibit 4-13B shows a minimal outer separation that may be applicable to 
ground-level freeways and high-speed arterial streets. This outer separation consists simply of the 
shoulders of the through roadway and the frontage road, as well as a physical barrier. 
Exhibit 4-13C shows a depressed arterial with a cantilevered frontage road. In this example, the 
inside edge of the frontage road is located directly over the outside edge of the through roadway. 
Exhibit 4-1 3D illustrates a common type of outer separation along a section of depressed 
freeway, Exhibit 4-13E shows a walled section at a depressed arterial with a ramp, and 
Exhibit 4-13F shows a typical freeway outer separation with a ramp. 



NOISE CONTROL 
General Considerations 

Noise may be defined as unwanted sound. Motor vehicles generate traffic noise from the 
motor, aerodynamics, exhaust, and interaction of the tires with the roadway. Efforts should be 
made to minimize the radiation of noise into noise-sensitive areas along the highway. The 
designer should evaluate existing or potential noise levels and estimate the effectiveness of 
reducing highway traffic noise through location and design considerations. 




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Exhibit 4-13, Typical Outer Separations 



The physical measurement of human reaction to sound is difficult because there is no 
instrument that will measure this directly. A close correlation can be obtained by using the 
A-scale on a standard sound level meter. The meter yields a direct reading in effective decibels 
(dBA). 



348 



Cross Section Elements 



A few general relationships may be helpful in understanding some of the principles of sound 
generation and transmission. Because noise is measured on a logarithmic scale, a decrease of 
10 dBA will appear to an observer as only half of the original noise level. For example, a noise of 
70 dBA sounds only one-half as loud as 80 dBA, assuming the same frequency composition and 
other things being equal. A doubling of the noise source produces a 3 dBA increase in the noise 
level. For example, if a single vehicle produces a noise level of 60 dBA at a certain distance from 
the receiver, two of these vehicles at a common point of origin will produce 63 dBA, four 
vehicles will produce 66 dBA, eight vehicles will produce 69 dBA, and so forth. 

Noise decreases with distance, but not as quickly as one might expect. For example, the 
sound level will decrease approximately 3 to 4.5 dBA for each doubling of distance from a 
highway. 

The same traffic noise level will produce different human reactions depending on the 
environment in which the noise is heard. The actual noise level is not, in itself, a good predictor 
of public annoyance. For example, the reaction is usually less if the noise source is hidden from 
view. The type of development in an area is another factor that affects the annoyance level. High 
traffic noise levels are usually more tolerable in industrial than in residential areas. Other factors 
that influence human reactions to noise are pitch and intermittency. The higher the pitch or more 
pronounced the intermittency of the noise, the greater the degree of annoyance. For further 
information, see the AASHTO Guide on Evaluation and Abatement of Traffic Noise (32). 



General Design Procedures 

The first step in analyzing the effects of noise from a proposed highway facility is to define 
the criteria for noise impacts. With these criteria defined, the location of noise-sensitive areas can 
be identified. These may include residential areas, schools, churches, motels, parks, hospitals, 
nursing homes, libraries, etc. The existing noise levels are determined by measurement of 
identified noise-sensitive land uses or acdvities. 

The highway-generated noise level is then predicted by one of the noise prediction methods 
presently available. Pertinent factors are traffic characteristics (speed, volume, and composition), 
topography (vegetation, barriers, and distance), and roadway characteristics (configuration, 
pavement type, grades, and type of facility). The prediction is normally based on the highway 
traffic that will yield the worst hourly traffic noise on a regular basis for the design year. More 
detailed information on noise prediction is available (17, 18, 19, 20, 21). 

Exhibit 4-14 provides FHWA noise-abatement criteria for various land uses. These sound 
levels are used to determine the noise impact on each land use. Traffic noise impacts occur under 
two criteria: 1) when the predicted levels approach or exceed the noise-abatement criteria and, 
2) when predicted noise levels substantially exceed the existing noise level, even though the 
predicted levels may not exceed the noise-abatement criteria. To adequately assess the traffic 
noise impact of a proposed project, both criteria should be analyzed. 



349 



AASHTO — Geometric Design of Highways and Streets 



Activity 
category 



Design noise levels 
(dBA)^ 



Category description 



Leq(h)^ 



Lio(h) 



A Tracts of land in which serenity and quiet are of 57 60 

extraordinary significance and serve an important (Exterior) 

public need and where the preservation of those 

qualities is essential if the area is to continue to serve 

its intended purpose. Such areas could include 

amphitheaters, particular parks or portions of parks, 

open spaces or historic districts which are dedicated 

or recognized by appropriate local officials for 

activities requiring special qualities of serenity and 

quiet. 
B Picnic areas, recreation areas, playgrounds active 67 70 

sports areas and parks not included in Category A (Exterior) 

and residences, motels, hotels, public meeting 

rooms, schools, churches, libraries, and hospitals. 
C Developed lands, properties or activities not included 72 75 

in Categories A or B above. (Exterior) 

D Undeveloped lands which do not contain -'' 

improvements or activities devoted to frequent 

human habitation or use and for which such 

improvements or activities are unplanned and not 

programmed. 
E Residences, motels, hotels, public meeting rooms, 52 55^^ 

schools, churches, libraries, hospitals, and (Interior) 
auditoriums. ^_^ 

Source: Federal Aid Highway Program Manual, Vol. 7, Ch. 7, Sec. 3 Transmittal 348, August 9, 
1982. 

Either Lio(h) or Leq(h) (but not both) may be used for a specific project. 

Noise-abatement criteria have not been established for these lands. They may be treated as 
developed lands if the probability for development is high. Provisions for noise abatement 
would be based on the need, expected benefits, and costs of such measures. 

Interior noise abatement criteria in this category apply to (1) indoor activities where no extreme 
noise-sensitive land use or activity is identified, and (2) exterior activities that are either remote 
from the highway or shielded so that they will not be significantly affected by the noise, but the 
interior activities will. 



Exhibit A-M. Noise- Abatement Criteria for Various Land Uses 



Noise Reduction Designs 

Potential noise problems should be identified early in the design process. Line, grade, 
earthwork balance, and right-of-way should all be worked out with noise in mind. Noise 
attenuation may be inexpensive and practical if built in the design and expensive if not considered 
until the end of the design process. An effective method of reducing traffic noise from adjacent 
areas is to design the highway so that some form of soUd material blocks the line of sight between 
the noise source and the receptors. Advantage should be taken of the terrain in forming a natural 
barrier so that the appearance remains aesthetically pleasing. 



350 



Cross Section Elements 



In terms of noise considerations, a depressed highway section is the most desirable. 
Depressing the roadway below ground level has the same general effect as erecting barriers (i.e., 
a shadow zone is created where noise levels are reduced [see Exhibit 4-15]). Where a highway is 
constructed on an embankment, the embankment beyond the shoulders will sometimes block the 
line of sight to receptors near the highway, thus reducing the potential noise impacts (see 
Exhibit 4-16). 

Special sound barriers may be justified at certain locations, particularly along ground»Ievel 
or elevated highways through noise-sensitive areas. Concrete, wood, metal, or masonry walls are 
very effective. One of the more aesthetically pleasing barriers is the earth berm that has been 
graded to achieve a natural form blending with the surrounding topography. The practicality of 
berm construction should be considered as part of the overall grading plan for the highway. There 
will be instances where an effective earth berm can be constructed within normal right-of-way or 
with a minimal additional right-of-way purchase. If right-of-way is insufficient to accommodate a 
full-height earth berm, a lower earth berm can be constructed in combination with a wall or 
screen to achieve the desired height. 



Shrubs, trees, or ground covers are not very efficient in shielding sound because of their 
permeability to air flow. However, almost all buffer plantings offer some noise reduction and 
exceptionally wide and dense plantings may result in substantial reductions in noise levels. Even 
where the noise reduction is not considered significant, the aesthetic effects of the plantings will 
produce a positive effect. 




DEPRESSED (SLOPING SIDE WALLS) 



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Exhibit 4-15» Effects of Depressing the Highway 



351 



AASHTO — Geometric Design of Highways and Streets 




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Exhibit 4-16, Effects of Elevating the Highway 

ROADSIDE CONTROL 
General Considerations 

The efficiency and safety of a highway without control of access depend greatly upon the 
amount and character of roadside interference, characterized by vehicle movements to and from 
businesses, residences, or other development along the highway. Abutting property owners have 
rights of access, but it is desirable that the highway authority be empowered to regulate and 
control the location, design, and operation of access driveways and other roadside elements such 
as mailboxes. Such access control minimizes interference to through traffic on the highway. 
Interference resulting from indiscriminate roadside development and uncontrolled driveway 
connections results in lowered capacity, increased conflict, and early obsolescence of the 
highway. 



Driveways 

Driveway terminals are, in effect, low-volume intersections; thus, their design and location 
merit special consideration. The operational effects of driveways are directly related to the 
functional classification of the roadway to which they provide access. For example, whereas 
driveways might adversely affect the operation of arterials, they become important links on local 
streets that provide access to local establishments. 



352 



Cross Section Elements 



Driveways used for right turns only are desirable where the cross section includes a curbed 
median or a flush median and median barrier. Driveways used for both right and left turns offer 
considerably more interference to through traffic and are undesirable on arterial streets. However, 
on major streets with numerous motorist-oriented businesses, the elimination of left turns at 
driveways may worsen traffic operations by forcing large volumes of traffic to make U-turns or 
travel around the block in order to reach their destination. 

The regulation and design of driveways is intimately linked with the available right-of-way 
and the land use and zoning control of the adjacent property. On new facilities, the needed right- 
of-way can be obtained to provide the desired degree of driveway regulation and control. To 
prohibit undesirable access conditions on existing facilities, either additional right-of-way can be 
acquired or agreements can be made with property owners to improve existing conditions. Often 
the desired degree of driveway control must be effected through the use of police powers by 
requiring permits for all new driveways and adjustment of existing that do not conform to 
established regulations. The objective of driveway regulations is to preserve efficiency and 
promote operational efficiency by prescribing desirable spacing and proper layout of driveways. 
The attainment of these objectives is dependent upon the type and extent of legislative authority 
granted to the highway agency. Many states and local municipalities have developed design 
policies for driveways and formed separate units to issue permits for new, or for changes in 
existing, driveway connections to main highways. For further information on the regulation and 
design of driveways, refer to Guidelines for Driveway Design and Location (22). 

Driveway regulations generally control right-of-way encroachment, driveway location, 
driveway design, sight distance, drainage, use of curbs, parking, setback, lighting, and signing. 
Some of the principles of intersection design can also be applied directly to driveways. An 
important feature of driveway design is the elimination of large graded or paved areas adjacent to 
the traveled way upon which drivers can enter and leave the facility at will. Another feature is the 
provision of adequate driveway widths, throat dimensions, and proper layout to accommodate the 
types of vehicles patronizing the roadside establishment. 

Sight distance, another important design control, can be limited by the presence of 
unnecessary roadside structures. Therefore, no advertising signs should be permitted in the right- 
of-way. Billboards or other elements outside the right-of-way that obstruct sight distance should 
be controlled by statutory authority or by purchase of easements. 

For roadways without access control but with concentrated business development along the 
roadside, consideration should be given to the use of a frontage road. This type of control and 
design is particularly pertinent to a main highway or street on a new location for which sufficient 
right-of-way can be acquired. In the first stage, intermittent sections of frontage roads are 
constructed to connect the few driveways initially needed. Then, in succeeding stages, extensions 
or additional sections of frontage roads are provided to intercept driveways resulting from further 
development of the roadsides. Thus, serious roadside interference is prevented at all stages, and 
the through character of the highway or street is preserved by gradual and judicious provision of 
frontage roads. 



353 



AASHTO — Geometric Design of Highways and Streets 



Mailboxes 

Mailboxes and appurtenant newspaper tubes served by carriers in vehicles may very well 
constitute a risk to motorists either directly or indirectly, depending upon the placement of the 
mailbox, the cross-section dimensions of the highway or street, sight distance conditions in the 
vicinity of the mailbox, traffic volume, and impact resistance of the mailbox support. The safety 
of both the carrier and the motoring public is affected whenever the carrier slows for a stop and 
then resumes travel along the highway. The risk is greatly increased if the cross section of the 
highway and the lateral placement of mailboxes are such that the vehicle occupies a portion of the 
traveled way while the mailbox is being serviced. 

The mounting height of the box places the box in a direct line with the windshield on many 
vehicles. This situation is more critical where multiple box installations are encountered. In many 
areas, the typical multiple mailbox installation consists of two or more posts supporting a 
horizontal member, usually a timber plank, which carries the group of mailboxes. The horizontal 
support element tends to penetrate the windshield and enter the passenger compartment when 
struck by a vehicle. Such installations are to be avoided where exposed to traffic. In fact, the 
mailbox and support should be, where practical, located in an area not exposed to through traffic. 

Mailboxes should be placed for maximum convenience to the patron, consistent with safety 
considerations for highway traffic, the carrier, and the patron. Consideration should be given to 
minimum walking distance within the roadway for the patron, available stopping sight distance in 
advance of the mailbox site (especially on older roads), and potential restriction to comer sight 
distance at driveway entrances. The placing of mailboxes along high-speed, high-volume 
highways should be avoided if other practical locations are available. New installations should, 
where practical, be located on the far right side of an intersection with a public road or private 
driveway entrance. Boxes should be placed only on the right-hand side of the highway in the 
direction of travel of the carrier except on one-way streets where they may also be placed on the 
left-hand side. 

Preferably, a mailbox should be placed so that it is not susceptible to being struck by an out- 
of -control vehicle. Where this placement is not practical, the supports should be of a type that will 
yield or break away safely if struck. The mailbox should be firmly attached to the support to 
prevent it from breaking loose and flying through the windshield. The same safety criteria also 
apply to multiple box installations. 

One of the primary considerations is the location of the mailbox in relation to the traveled 
way. Basically, a vehicle stopped at a mailbox should be clear of the traveled way. The higher the 
traffic volume or the speed, the greater the clearance should be. An exception to this may be 
considered on low-volume, low-speed roads and streets. 

Most vehicles stopped at a mailbox will be clear of the traveled way when the mailbox is 
placed outside a 2.4 m [8 ft] wide usable shoulder or turnout. This position is recommended for 
most rural highways. For high-volume, high-speed highways, it is recommended that the width of 
shoulder in front of the mailbox or turnout be increased to 3.0 m [10 ft] or even 3.6 m [12 ft] 
for some conditions. However, it may not be practical to consider even a 2.4-m [8-fl:] shoulder or 

354 



Cross Section Elements 



turnout on low-volume, low-speed roads or streets. To provide space for opening the mailbox 
door, it is recommended that the roadside face of a mailbox be set 200 to 300 mm [8 to 12 in] 
outside the shoulder or turnout. Current postal regulations should be consulted for specific set- 
back criteria. 

In areas of heavy or frequent snowfall, mailboxes may be placed at about the customary line 
of the plowed windrow, but no closer than about 3.0 m [10 ft] to the edge-of-traveled way if the 
shoulder is wider than 3.0 m [10 ft]. Cantilever mailbox supports may prove advantageous for 
snow-plowing operations. Wherever practical, mailboxes should be located behind existing 
guardrail. 

In some urban and suburban areas, mailboxes are located along selected streets and 
highways where the local post office has established delivery routes. In these areas when the 
roadway has a curb and gutter section, mailboxes should be located with the front of the box 150 
to 300 mm [6 to 12 in] back of the face of curb. On residential streets without curbs or shoulder 
and which carry low-traffic volumes operating at low speeds, the roadside face of a mailbox 
should be offset between 200 to 300 mm [8 to 12 in] behind the edge of the traveled way. 

For guidance on mailbox installations, refer to the latest editions of AASHTO's A Guide for 
Erecting Mailboxes on Highways (23) and Roadside Design Guide (10). 



TUNNELS 
General Considerations 

Development of streets or highways may include sections constructed in tunnels either to 
carry the streets or highways under or through a natural obstacle or to minimize the impact of the 
freeway on the community. General conditions under which tunnel construction may be 
warranted include: 

® Long, narrow terrain ridges where a cut section may either be costly or carry 

environmental consequences 
® Narrow rights-of-way where all of the surface area is needed for street purposes 
® Large intersection areas or a series of adjoining intersections on an irregular or diagonal 

street pattern 
® Railroad yards, airport runways, or similar facilities 
© Parks or similar land uses, existing or planned 
® Where right-of-way acquisition costs exceed cost of tunnel construction and operation. 

Although the costs of operation and maintenance of tunnels are beyond the scope of this 
policy, these costs should nevertheless be considered. 

Additional construction and design features of tunnel sections are discussed below. It is not 
intended that this section be considered complete on the subject of highway tunnels. Instead, the 
material that follows provides highway planners and designers with general background 

355 



AASHTO — Geometric Design of Highways and Streets 



information. To accomplish this basic objective, some simplification of subject matter is 
appropriate. As with any highly specialized branch of engineering, such simplified information 
should be used with caution. In addition, the ventilating, lighting, pumping, and other mechanical 
or electrical considerations in tunnel design are regarded as outside the scope of this policy. 



Types of Tunnels 

Tunnels can be classified into two major categories: (1) tunnels constructed by mdning 
methods, and (2) tunnels constructed by cut-and-cover methods. 

The first category refers to those tunnels that are constructed without removing the overlying 
rock or soil. Usually this category is subdivided into two very broad groups according to the 
appropriate construction method. The two groups are named to reflect the overall character of the 
material to be excavated: hard rock and soft ground. 

Of particular interest to the highway designer are the structural requirements of these 
construction methods and their relative costs. As a general rule, hard-rock tunneling is less 
expensive than soft-ground tunneling. A tunnel constructed through solid, intact, and 
homogeneous rock will normally represent the lower end of the scale with respect to structural 
demands and construction costs. A tunnel located below water in material needing immediate and 
heavy support will require extremely expensive soft-ground tunneling techniques such as shield 
and compressed air methods. 

The shape of the structural cross section of the tunnel varies with the type and magnitude of 
loadings. In those cases where the structure will be subjected to roof loads with little or no side 
pressures, a horseshoe-shaped cross section is used. As side pressures increase, curvature is 
introduced into the sidewalls and invert struts added. When the loadings approach a distribution 
similar to hydrostatic pressures, a full circular section is usually more efficient and economical. 
All cross sections are dimensioned to provide adequate space for ventilation ducts. 

The second category of tunnel classification deals with the two types of tunnels that are 
constructed from the surface: trench and cut-and-cover tunnels. The latter are used exclusively for 
subaqueous work. In the trench method, prefabricated tunnel sections are constructed in shipyards 
or dry docks, floated to the site, sunk into a dredged trench, and joined together underwater. The 
trench is then backfilled. When conditions are favorable with respect to subsurface soil, amount 
of river current, volume and character of river traffic, availability of construction facilities, and 
type of existing waterfront structures, the trench method may prove more economical than 
alternative methods. 

The cut-and-cover method is by far the most common type of tunnel construction for 
shallow tunnels, which often occurs in urban areas. As the name implies, the method consists of 
excavating an open cut, building the tunnel within the cut, and backfilling over the completed 
structure. Under ideal conditions, this method is the most economical for constructing tunnels 
located at a shallow depth. However, it should be noted that surface disruption and problems with 
utilities generally make this method very expensive and difficult. 

356 



Cross Section Elements 



General Design Considerations 

Tunnels should be made as short as practical because the feeling of confinement and 
magnification of traffic noise can be unpleasant to motorists, and tunnels are the most expensive 
highway structures to construct. The horizontal alignment through the tunnel is an important 
design consideration as well. Keeping as much of the tunnel length as practical on tangent will 
not only minimize the length but also improve operating efficiency. Tunnels designed with 
extreme curvature may result in limited stopping sight distance. Therefore, sight distance across 
the face of the tunnel wall should be carefully examined. 

The vertical alignment through the tunnel is another important design consideration. Grades 
in tunnels should be determined primarily on the basis of driver comfort while striving to reach a 
point of economic balance between construction costs and operating and maintenance expenses. 
Many factors have to be considered in tunnel lengths and grades and their effects on tunnel 
lighting and ventilation. For example, lighting expenses are highest near portals and depend 
heavily on availability of natural light and the need to make a good light transition. Ventilation 
costs depend on length, grades, natural and vehicle-induced ventilation, type of system, and air 
quality constraints. 

The overall roadway design should avoid the need for guide signs within tunnels, because 
normal vertical and lateral clearances are usually insufficient for such signing and additional 
clearance can be provided only at very great expense. Exit ramps should be located a sufficient 
distance downstream from the tunnel portal to permit needed guide signs between the tunnel and 
the point of exit. This distance should be a minimum of 300 m [1,000 ft]. It is also highly 
undesirable that traffic be expected to merge, diverge, or weave within a tunnel, as might be the 
case if the tunnel is located between two closely spaced interchanges. Therefore, forks and exit or 
entrance ramps should be avoided within tunnels. 



Tunnel Sections 

From the standpoint of service to traffic, the design criteria used for tunnels should not differ 
materially from those used for grade separation structures. The same design criteria for alignment 
and profile and for vertical and horizontal clearances generally apply to tunnels except that 
minimum values are typically used because of high cost and restricted right-of-way. 

Full left- and right-shoulder widths of the approach freeway desirably should be carried 
through the tunnel. Actually, the need for added lateral space is greater in tunnels than under 
separation structures because of the greater likelihood of vehicles becoming disabled in the longer 
lengths. If shoulders are not provided, intolerable delays may result when vehicles become 
disabled during periods of heavy traffic. However, the cost of providing shoulders in tunnels may 
be prohibitive, particularly on long tunnels that are constructed by the boring or shield-drive 
methods. Thus, the determination of the width of shoulders to be provided in a tunnel should be 
based on an in-depth analysis of all factors involved. Where it is not practical to provide 
shoulders in a tunnel, arrangements should be made for around-the-clock emergency service 
vehicles that can promptly remove any stalled vehicles. 

357 



AASHTO— Geometric Design of Highways and Streets 



Exhibit 4-17 illustrates the minimum and desirable cross sections for two-lane tunnels. The 
minimum roadway width between curbs, as shown in Exhibit 4-17A, should be at least 0.6 m 
[2 ft] greater than the approach traveled way, but not less than 7.2 m [24 ft]. The curb or sidewalk 
on either side should be a minimum of 0.5 m [1.5 ft]. The total clearance between walls of a two- 
lane tunnel should be a minimum of 9 m [30 ft]. The roadway width and the curb or sidewalk 
width can be varied as needed within the 9-m [30-ft] minimum wall clearance; however, each 
width should not be less than the minimum value stated above. 




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43 to4,9m*[14-16ft] 



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[1.5 ft] [1.5 ft] 



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[30 ft] 



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Exhibit 4-17. Typical Two-lane Tiinnel Sections 



358 



Cross Section Elements 



The minimum vertical clearance is 4.9 m [16 ft] for freeways and 4.3 m [14 ft] for other 
highways. However, the minimum clear height should not be less than the maximum height of 
load that is legal in a particular state, and it is desirable to provide an allowance for future 
repaving of the roadways. 

Exhibit 4-17B illustrates the desirable section with two 3.6-m [12-ft] lanes, a 3.0-m [10-ft] 
right shoulder, a 1.5-m [5-ft] left shoulder, and a 0.7-m [2.5-ft] curb or sidewalk on each side. 
The roadway width may be distributed to either side in a different manner if needed to better fit 
the dimensions of the tunnel approaches. The vertical clearance for the desirable section is 4.9 m 
[16 ft] for freeways and 4.3 m [14 ft] for other highways. 

Normally, pedestrians are not permitted in freeway tunnels; however, space should be 
provided for emergency walking and for access by maintenance personnel. Raised sidewalks, 
0.7 m [2.5 ft] wide, are desirable beyond the shoulder areas to serve the dual purpose of a safety 
walk and a buffer to prevent the overhang of vehicles from damaging the wall finish or the tunnel 
lighting fixtures. Separate tunnels may be warranted for pedestrians or other special uses, such as 
bicycle routes. 

Exhibit 4-18 shows several tunnel sections as well as a partially covered highway. 
Directional traffic should be separated for safety reasons and to relieve the dizzying effect of two- 
way traffic in a confined space. This separation can be achieved by providing a twin opening as 
shown in Exhibit 4- 18 A, by multilevel sections as shown in Exhibits 4-18B and 4-18C, or by 
terraced structures as shown in Exhibit 4-18D. The terraced roadways are open on the outside for 
light, view, and ventilation. Exhibit 4-1 8E illustrates roadways that are tunneled under hillside 
buildings. A partially covered section, as shown in Exhibit 4-18F, provides light and ventilation to 
the motorist while minimizing freeway intrusion on the community traversed. This type of cross 
section is covered in the section ''Depressed Freeways*' in Chapter 8. 



Examples of Tunnels 

Exhibit 4-19 shows a freeway tunnehng through a hillside. The portals are staggered and 
attractively designed. The interchange is located a sufficient distance from the tunnel to allow 
space for effective signing and the necessary traffic maneuvers. 

Exhibit 4-20 illustrates the interior of a three-lane directional tunnel. Note the two rows of 
lighting fixtures on each wall in the foreground. The upper row of lights provides supplemental 
daytime lighting at the entrance portal to reduce the optical shock of traveling from natural to 
artificial lighting. The ceramic-tile finish on the walls and ceiling provides reflective surfaces that 
increase the brightness level and uniformity of lighting. A curb-to-curb width of 12.3 m [41 ft] is 
provided with 0.7 m [2.5 ft] wide safety walks along each wall. 



359 



AASHTO — Geometric Design of Highways and Streets 



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Exhibit 448, Diagrammatic TuiiEel Sections 




Exhibit 4-19, Entrance to a Freeway Tunnel 



360 



Cross Section Elements 




Exhibit 4-20. Interior of a S-lane One-way Tennel 

PEDESTRIAN FACILITIES 

Sidewalks 

Sidewalks are an integral part of city streets but are rarely provided in rural areas. However, 
the potential for collisions with pedestrians is higher in many rural areas due to the higher speeds 
and general absence of lighting. The limited data available suggest that sidewalks in rural areas 
do reduce pedestrian collisions. 

Sidewalks in rural and suburban areas are more often justified at points of community 
development, such as residential areas, schools, local businesses, and industrial plants, that result 
in pedestrian concentrations near or along the highways. When suburban residential areas are 
developed, initial roadway facilities are needed for the development to function, but the 
construction of sidewalks is sometimes deferred. However, if pedestrian activity is anticipated, 
sidewalks should be included as part of the construction. Shoulders may obviate the need for 
sidewalks if they are of a type that encourages pedestrian use in all weather conditions. If 
sidewalks are utilized, they should be separated from the shoulder. If the sidewalk is raised above 
the level of the shoulder, the cross section typically approaches that of an urban highway. 

In suburban and urban locations, a border area generally separates the roadway from a 
community's homes and businesses. The main function of the border is to provide space for 
sidewalks. Other functions are to provide space for streetlights, fire hydrants, street hardware, and 
aesthetic vegetation and to serve as a buffer strip. Border width varies considerably, but 2.4 m 



361 



AASHTO — Geometric Design of Highways and Streets 



[8 ft] is considered an appropriate minimum width. Swale ditches may be located in these borders 
to provide an economical alternative to curb and gutter sections. 

Sidewalk widths in residential areas may vary from 1.2 to 2.4 m [4 to 8 ft]. The width of a 
planted strip between the sidewalk and traveled-way curb, if provided, should be a minimum of 
0.6 m [2 ft] to allow for maintenance activities. Sidewalks covering the full border width are 
generally justified and often appropriate in conHumercial areas, through adjoining multiple- 
residential complexes, near schools and other pedestrian generators, and where border width is 
restricted. 

Where sidewalks are placed adjacent to the curb, the widths should be approximately 0.6 m 
[2 ft] wider than those widths used when a planted strip separates the sidewalk from the curb. 
This additional width provides space for roadside hardware and snow storage outside the width 
needed by pedestrians. It also allows for the proximity of moving traffic, the opening of doors of 
parked cars, and bumper overhang on angled parking. 

Justification for the construction of sidewalks depends upon the potential for vehicle- 
pedestrian conflicts. Traffic volume-pedestrian warrants for sidewalks along highways have not 
been established. In general, wherever roadside and land development conditions affect regular 
pedestrian movement along a highway, a sidewalk or path area, as suitable to the conditions, 
should be furnished. 

As a general practice, sidewalks should be constructed along any street or highway not 
provided with shoulders, even though pedestrian traffic may be light. Where sidewalks are built 
along a high-speed highway, buffer areas should be estabhshed so as to separate them from the 
traveled way. 

Sidewalks should have all-weather surfaces to ensure their intended use. Without them, 
pedestrians often choose to use the traveled way. Pedestrian crosswalks are regularly marked in 
urban areas but are rarely marked on rural highways. However, where there are pedestrian 
concentrations, appropriate traffic-control devices should be used, together with appropriate 
walkways constructed within the right-of-way. 

When two urban communities are in proximity to one another, consideration should be given 
to connecting the two communities with sidewalks, even though pedestrian traffic may be light. 
This may avoid driver-pedestrian conflicts on these sections of a through route. 

Pedestrian facilities such as sidewalks must be designed to accommodate persons with 
disabilities. See the sections on "Grade-separated Pedestrian Crossings" and "Sidewalk Curb 
Ramps" later in this chapter for further discussion on this point. 

Generally, the guidelines set forth in this section for the accommodation of pedestrians are 
also applicable to bridges. However, because of the high cost of bridges and the operational 
features that may be unique to bridge sites, pedestrian-way details on a bridge will often differ 
from those on its approaches. For example, where a planted strip between a sidewalk and the 



362 



Cross Section Elements 



traveled way approaches a bridge, continuation of the offset, affected by the planted strip, will 
seldom be justified. 

Where flush shoulders approach a bridge and light pedestrian traffic is anticipated on the 
shoulders, the shoulder width should be continued across the bridge, and possibly increased, to 
account for the restriction to pedestrian escape imposed by the bridge rail. A flush roadway 
shoulder should not be interrupted by a raised walkway on a bridge. Where such installations 
already exist, and removal is not economically justified, the ends of the walkway should be 
ramped into the shoulder at a rate of approximately 1:20 with the shoulder grade. 

Provisions for pedestrians are often appropriate on street overcrossings and on longer bridge 
crossings. On lower-speed streets, a vertical curb at the edge of the sidewalk is usually sufficient 
to separate pedestrians from vehicular traffic. Continuity of curb height should be maintained on 
the approaches to and over structures. For higher-speed roadways on structures, a barrier-type rail 
of adequate height may be used to separate the walkway and the traveled way. A pedestrian-type 
rail or screen should be used at the outer edge of the walkway. On long bridges (greater than 60 m 
[200 ft]), a single walkway may be provided. However, care should be taken to ensure that 
approach walkways provide safe and relatively direct access to the bridge walkway. Fences may 
need to be erected to channelize pedestrians and prevent or control conflicts between pedestrians 
and vehicular traffic. 

For a discussion of the potential problems associated with the introduction of a traffic barrier 
between a roadway and a walkway, see the section on "Bridge Railings," earlier in this chapter. 
For a discussion on designing sidewalks to accommodate persons with disabihties, see the section 
on "Sidewalk Curb Ramps," later in this chapter. Further guidance on sidewalk and pedestrian 
crossing design can be found in current Americans with Disabilities Act Accessibility Guidelines 
(ADAAG) (24) and in the AASHTO Guide for the Planning, Design, and Operation of 
Pedestrian Facilities (25). 



Grade-Separated Pedestrian Crossings 

A grade-separated pedestrian facility allows pedestrians and motor vehicles to cross at 
different levels, either over or under a roadway. It provides pedestrians with a safe refuge for 
crossing the roadway without vehicle interference. Pedestrian separations should be provided 
where pedestrian volume, traffic volume, intersection capacity, and other conditions favor their 
use, although their specific location and design require individual study. They may be warranted 
where there are heavy peak pedestrian movements, such as at central business districts, factories, 
schools, or athletic fields, in combination with moderate to heavy vehicular traffic or where 
unusual risk or inconvenience to pedestrians would otherwise result. Pedestrian separations, 
usually overpasses, may be needed at freeways or expressways where cross streets are terminated. 
On many freeways, highway overpasses for cross streets may be limited to three- to five-block 
intervals. As this situation imposes an extreme inconvenience on pedestrians desiring to cross the 
freeway at the terminated streets, pedestrian separations may be provided. Local, State, and 
Federal laws and codes should be consulted for possible additional criteria concerning need, as 
well as additional design guidance. 

363 



AASHTO — Geometric Design of Highways and Streets 



Where there are frontage roads adjacent to the arterial highway, the pedestrian crossing may 
be designed to span the entire or only the through roadway. Separations of both through roadways 
and frontage roads may not be justified if the frontage roads carry light and relatively slow- 
moving traffic; however, in some cases the separation should span the frontage roads as well. 
Fences may be needed to prevent pedestrians from crossing the arterial at locations where a 
separation is not provided. 

Pedestrian crossings or overcrossing structures at arterial streets are not likely to be used 
unless it is obvious to the pedestrian that it is easier to use such a facility than to cross the traveled 
way. Generally, pedestrians are more reluctant to use undercrossings than overcrossings. This 
reluctance may be minimized by locating the undercrossing on line with the approach sidewalk 
and ramping the sidewalk gently to permit continuous vision through the undercrossing from the 
sidewalk. Good sight lines and lighting are needed to enhance a sense of security. Ventilation 
may be needed for very long undercrossings. 

Pedestrian ramps should be provided at all pedestrian separation structures. Where warranted 
and practical, a stairway can be provided in addition to the ramp. Elevators should be considered 
where the length of ramp would result in a difficult path of travel for a person with or without a 
disability. 

Walkways for pedestrian separations should have a minimum width of 2.4 m [8 ft]. Greater 
widths may be needed where there are exceptionally high volumes of pedestrian traffic, such as in 
the downtown areas of large cities and around sports stadiums. 

A serious problem associated with pedestrian overcrossings and highway overpasses with 
sidewalks is vandals dropping objects into the path of traffic moving under the structure. The 
consequences of objects being thrown from bridges can be very serious. In fact, there are frequent 
reports of fatalities and major injuries caused by this type of vandalism. There is no practical 
device or method yet devised that can be universally applied to prevent a determined individual 
from dropping an object from an overpass. For example, small objects can be dropped through 
mesh screens. A more effective deterrent is a solid plastic enclosure. However, these are 
expensive and may be insufferably hot in the summer. They also obscure and darken the 
pedestrian traveled way, which may be conducive to other forms of criminal activity. Any 
completely enclosed pedestrian overpass has an added problem that children may walk or play on 
top of the enclosure. In areas subject to snow and icing conditions, the possibility that melting 
snow and ice may drop from the roof of a covered overpass and fall onto the roadway below 
should be considered. 

At present it is not practical to establish absolute warrants as to when or where barriers 
should be installed to discourage the throwing of objects from structures. The general need for 
economy in design and the desire to preserve the clear lines of a structure unencumbered by 
screens should be carefully balanced against the need to provide safe operations for both 
motorists and pedestrians. 

Locations where screens definitely should be considered at the time the overpass is 
constructed include: 

364 



Cross Section Elements 



® On an overpass near a school, a playground, or elsewhere where it would be 

expected that the overpass would be frequently used by children unaccompanied 

by adults. 
® On all overpasses in large urban areas used exclusively by pedestrians and not 

easily kept under surveillance by police. 
@ On an overpass where the history of incidents on nearby structures indicates a need 

for screens. 

Screens should also be installed on existing structures where there have been incidents of 
objects being dropped from the overpass and where it seems evident that increased surveillance, 
warning signs, or apprehension of a few individuals involved will not effectively alleviate the 
problem. 

More complete information on the use of protective screens on pedestrian overpasses is 
available in the AASHTO Roadside Design Guide (10). 

Exhibit 4-21 illustrates two typical pedestrian overcrossings of major highways. 



Sidewalk Curb Ramps 

When designing a project that includes curbs and adjacent sidewalks, proper attention should 
be given to the needs of persons with disabilities whose means of mobility are dependent upon 
wheelchairs and other devices (26). The street intersection with steep-faced curbs need not be an 
obstacle to persons with disabilities. In fact, adequate and reasonable access can be provided for 
sidewalk curb ramps. 

Design details of sidewalk and wheelchair curb ramps will vary in relation to the following 
factors: 

^ Sidewalk width 

® Sidewalk location with respect to the back face of curb 

® Height and width of curb cross section 

® Design turning radius and length of curve along the curb face 

® Angle of street intersections 

® Planned or existing location of sign and signal control devices 

® Storm water inlets and public service utilities 

® Potential sight obstructions 

® Street width 

® Border width 

@ Roadway grade in combination with the grades of the sidewalk, curb, ramps and 
gutter 



365 



AASHTO — Geometric Design of Highways and Streets 




■ M ■ M ■ ■ ■ y ^ m " ^ y " ^* iwm»»»M ■■■■ ...^..^ 




Exhibit 4-21 o Typical Pedestrian Overpasses on Major Highways 



366 



Cross Section Elements 



As a result, basic curb ramp types have been established and used in accordance with the 
geometric characteristics of each intersection. Currently, ADAAG (24) requires a 0.9-m [3-ft] 
minimum curb ramp width and an 8.33 percent maximum grade. Cross slopes on adjacent 
sidewalks should be no greater than 2 percent. A level landing area is required at the top of each 
curb ramp. In addition, 0.6-m [2-ft] detectable warning strips that comply with ADAAG are 
recommended at the bottom of curb ramps to improve detectability by people with visual 
impairments. 

Exhibit 4-22 illustrates various sidewalk curb ramp designs. Exhibit 4-22A shows the 
condition where the entire grade differential is totally achieved outside the sidewalk. This 
condition is desirable since it does not require anyone to walk across the ramped area. In this 
case, a steep face curb can be used along the curb ramp if the presence of landscaping or other 
fixed obstructions constrain pedestrians from walking across the curb ramp. 

In most areas where sidewalks are needed, the curb ramp should be incorporated in the 
sidewalk, as shown in Exhibits 4-22B and 4~22C. Exhibit 4-22B reflects a normal design with 
adequate room for curb ramp slope development. Exhibit 4-22C shows an example where a width 
restriction results in the curb ramp being constructed totally within the sidewalk area. 

When other options are not practical, a built-up curb ramp, such as the one illustrated in 
Exhibit 4-22D, may be desirable. However, the curb ramp should not project into the traveled 
way. Also, drainage may be adversely affected if not properly considered. The curb ramp area 
should be protected and should only be used at locations that include a parking lane. 

The location of the sidewalk curb ramp should be carefully coordinated with respect to the 
pedestrian crosswalk lines. This planning should ensure that the bottom of the curb ramp is 
situated within the parallel boundaries of the crosswalk markings. The bottom of the curb ramp 
should be perpendicular to the face of the curb with the least amount of warping in the sidewalk, 
curb ramp, and street transitions. 

Exhibit 4-23 shows a typical sidewalk ramp at the middle of the curb radius. In areas where 
pedestrian and/or vehicular traffic volumes are moderate to high, use of this configuration should 
be discouraged. Such placement forces the curb ramp users to enter diagonally into the 
intersection, perhaps misdirecting them and exposing them to conflicts with traffic from two 
directions. This situation is of special concern to people who are visually impaired. 

Exhibit 4-24 illustrates sidewalk curb ramps at the beginning and end points of the curb 
radius. 

Curb ramps for persons with disabilities are not limited to intersections and marked 
crosswalks. Curb ramps should also be provided at other appropriate or designated points of 
pedestrian concentration, such as loading islands and midblock pedestrian crossings. Because 
non-intersection pedestrian crossings are generally unexpected by the motorist, warning signs 



367 



AASHTO — Geometric Design of Highways and Streets 








BACK CDGF / a 
OF SIDEWALK-^ 

ELEVATION VIEW OF fe 



-DO MOT PnOJECr 
INTO VEHICLi LAN^$ 



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*GONa StDEWALK 

SECTION A--A 



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Exhibit 4-22. Midblock Sidewalk Curb Ramp Details 



368 



Cross Section Elements 




Exhibit 4-23. Sidewalk Cerb Ramp at Middle of Radius — Discouraged Where Pedestrian 
and/or Vehicular Volumes are Moderate to High 



should be installed and parking should be prohibited to ensure adequate visibility. For additional 
design guidance and recommendations with respect to pedestrian crosswalk markings, refer to the 
MUTCD (8), ADAAG (24), and AASHTO Guide for the Planning, Design, and Operation of 
Pedestrian Facilities (25). 

Exhibit 4-25 shows a midblock sidewalk curb ramp. Curb ramps should have a non-skid 
surface. 

As shown in Exhibit 4-26, when a major highway or secondary intersecting road involves 
pedestrian traffic and the roadway geometries involve convex islands or median dividers, the plan 
should include curb ramps for persons with disabilities. People in wheelchairs cannot safely take 
refuge on islands that are less than 1.2 m [4 ft] wide because of their vulnerability to moving 
traffic. A 1.8-m [6-ft] island width is desirable. 



Each intersection will differ with respect to the intersection angles, turning roadway widths, 
size of islands, drainage inlets, traffic-control devices, and other variables previously described. 
An appropriate plan should be prepared that indicates all of the desired geometries, including 
vertical profiles at the curb flow line. The plan should then be evaluated to determine convenient 

369 



AASHTO — Geometric Design of Highways and Streets 



ininnniiiimmiiffl 




Exhibit 4-24, Sidewalk Curb Ramp at End of Curb Radius 




Exhibit 4-25, Sidewalk Curb Ramp at Midblock 



and safe locations of the ramps to accommodate usage by persons with disabilities. Drainage 
inlets should be located on the upstream side of all crosswalks and sidewalk ramps. This design 
operation will govern the pedestrian crosswalk patterns, stop bar locations, regulatory signs, and, 
in the case of new construction, establish the most desirable location of signal supports. 

Curb ramps should be provided at all intersections where curb and sidewalk are provided 
even though the highway grade may exceed the allowable sidewalk grade. This provision allows 
for wheelchairs to be easily maneuvered. For further information on sidewalk curb ramps for 
persons with disabilities, see the current ADAAG (24), the AASHTO Guide for the Planning, 
Design, and Operation of Pedestrian Facilities (25), and Designing Sidewalks 



370 



Cross Section Elements 




136 in] 



ExMMt 4-26» Median aed Island Openings 



and Trails for Access, Part I: Review of Existing Guidelines and Practices (27) and Part II: Best 
Practices Design Guide (28). 



BICYCLE FACIUTIES 

Most of the facilities needed for bicycle travel can consist of the street and highway system 
generally as it presently exists. However, at certain locations, or in certain conidors, it is 
appropriate to supplement the existing highway system by providing specifically designated 
bikeways. 

Provisions for bicycle facilities should be in accordance with the AASHTO Guide for the 
Development of Bicycle Facilities (7). Even if specific bicycle facilities are not provided, 
consideration should be given to other practical measures for enhancing bicycle travel on the 
highway. 

Chapter 2 provides further discussion on the subject of bicycle facilities. 



371 



AASHTO — Geometric Design of Highways and Streets 



BUS TURNOUTS 

Bus travel is an increasingly important mode of mass transportation. Bus turnouts serve to 
remove the bus from the traveled way. The location and design of turnouts should provide ready 
access in the safest and most efficient manner practical. 



Freeways 

The basic design objective for a freeway bus turnout is that the deceleration, standing, and 
acceleration of buses take place on pavement areas clear of and separated from the traveled way. 
Other elements in the design of bus turnouts include passenger platforms, ramps, stairs, railings, 
signs, and markings. Speed-change lanes should be long enough to enable the bus to leave and 
enter the traveled way at approximately the average running speed of the highway without undue 
discomfort to passengers. The length of acceleration lanes from bus turnouts should be well 
above the normal minimum values, as the buses start from a standing position and the loaded bus 
has a lower acceleration capability than passenger cars. Normal-length deceleration lanes are 
suitable. The width of the bus standing area and speed-change lanes, including the shoulders, 
should be 6.0 m [20 ft] to permit the passing of a stalled bus. The pavement areas of turnouts 
should contrast in color and texture with the traveled way to discourage through-traffic from 
encroaching on or entering the bus stop. 

The dividing area between the outer edge of freeway shoulder and the edge of bus turnout 
lane should be as wide as practical, preferably 6.0 m [20 ft] or more. However, in extreme cases, 
this width could be reduced to a minimum of 1.2 m [4 ft]. A barrier is usually needed in the 
dividing area, and fencing is desirable to keep pedestrians from entering the freeway. Pedestrian 
loading platforms should not be less than 1.5 m [5 ft] wide and preferably 1.8 m to 3.0 m 
[6 to 10 ft] wide. Some climates may warrant the covering of platforms. Exhibit 4-27 illustrates 
typical cross sections of turnouts including a normal section, a section through an underpass, and 
a section on an elevated structure. 



Arterials 

The interference between buses and other traffic can be considerably reduced by providing 
turnouts on arterials. It is somewhat rare that sufficient right-of-way is available on the lower type 
arterial streets to permit turnouts in the border area, but advantage should be taken of every 
opportunity to do so. 

To be fully effective, bus turnouts should incorporate (1) a deceleration lane or taper to 
permit easy entrance to the loading area, (2) a standing space sufficiently long to accommodate 
the maximum number of vehicles expected to occupy the space at one time, and (3) a merging 
lane to enable easy reentry into the traveled way. 

The deceleration lane should be tapered at an angle flat enough to encourage the bus 
operator to pull completely clear of the through lane before stopping. Usually it is not practical to 

372 



Cross Section Elements 




Barrier and F«nce Preferred 



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[2 ft] 



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Barrief and Fence Preferred 



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Bus Uil6 6 m Nlil»imiltil 



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Exhibit 4-27, Bus Tiireoiits 



provide a length sufficient to permit deceleration from highway speeds clear of the traveled way. 
A taper of about 5:1, longitudinal to transverse, is a desirable minimum. When the bus stop is on 
the far side of an intersection, the intersection area may be used as the entry area to the stop. 

The loading area should provide about 15 m [50 ft] of length for each bus. The width should 
be at least 3.0 m [10 ft] and preferably 3.6 m [12 ft]. The merging or reentry taper may be 
somewhat more abrupt than the deceleration taper but, preferably, should not be sharper than 3:1. 
Where the turnout is on the near side of an intersection, the width of cross street is usually great 
enough to provide the needed merging space. 

The minimum total length of turnout for a two-bus loading area should be about 55 m 
[180 ft] for a midblock location, 45 m [150 ft] for a near-side location, and 40 m [130 ft] for a far- 
side location. These dimensions are based on a loading area width of 3.0 m [10 ft]. They should 
be increased by 4 to 5 m [13 to 16 ft] for a width of 3.6 m [12 ft]. Greater lengths of bus turnouts 



373 



AASHTO — Geometric Design of Highways and Streets 



expedite bus maneuvers, encourage full compliance on the part of bus drivers, and lessen 
interference with through traffic. 

Exhibit 4-28 shows a bus turnout at a midblock location. The width of the turnout is 3.0 m 
[10 ft], and the length of the turnout, including tapers, is 63 m [210 ft]. The deceleration and 

acceleration tapers are 4: 1. 

For more information on bus turnouts, see the AASHTO Guide for Design of 
High-Occupancy Vehicle and Public Transportation Facilities (29) and Guidelines for the 
Location and Design of Bus Stops (30). 




E 



/ 



Jit. 



tisvmT^wm.m^K 



Exhibit 4-28, Midblock Bus Tumoiit 



Park-and-Rlde Facilities 



Location 



Park-and-ride facilities should be located adjacent to the street or highway and be visible to 
the commuters whom they are intended to attract. Preferably, the parking areas should be located 
at points that precede the bottlenecks or points where there is significant traffic congestion. They 
should be located as close to residential areas as practical in order to minimize travel by vehicles 

374 



Cross Section Elements 



with only one occupant and should be located far enough out that land costs are not prohibitive. 
In addition, bicycle and pedestrian access to park-and-ride facilities should be considered. 

Other considerations that affect parking lot location are impacts on surrounding land uses, 
available capacity of the highway system between the roadway and proposed sites, terrain, and 
the costs to acquire the land. 



Design 

The size of the park-and-ride parking lot is dependent upon the design volume, the available 
land area, and the size and number of other parking lots in the area. Twenty to sixty spaces 
represent a reasonable range. 

Each parking area should provide a drop-off facility close to the station entrance, plus a 
holding or short-term parking area for passenger pickup. This area should be clearly separated 
from the park-and-ride areas. 

Consideration should be given to the location for bus loading and unloading, taxi service, 
bicycle parking, and special parking for persons with disabilities. Conflicts between pedestrians 
and ve